School-Safe Puzzle Games

Jack & The Beanstalk


Here is the 3rd logic puzzle:

Jack has just swapped a cow for a magic seed. So he sows it and in just 1 second the seed grows into a massive beanstalk 100 meters high and makes a pause.

Throughout the next second, Jack begins to climb the beanstalk and he can only make 1 meter before the plant shakes and grows another 100 meters. It pauses again.

At this point it must be explained that the plant grows uniformly in all its length, precisely as if it was perfectly elastic and was being stretched from both ends. So, every point of the plant grows/stretches.

Throughout the following second, Jack climbs 1 meter and afterward the plant shakes and grows another 100 meters. It pauses. This process repeats indefinitely.

Will Jack ever reach the top of the beanstalk?

33 Comments to “Jack & The Beanstalk”

  1. hex | PUZZLE MASTER | Profile

    Let’s consider steps (corresponding to time in seconds) where L is the height of the beanstalk, and H is Jack’s height:
    L0 = 100
    H0 = 1

    Li+1 = Li + 100
    Hi+1 = Li+1 / Li * Hi + 1
    Note: In each step the beanstalk elongates, then Jack climbs up 1 meter

    We can prove by recursion that:
    Li = 100 * (i + 1)
    Hi = (i + 1) * Sum(1 / j) for j = 1 to i + 1

    The question then boils down to:
    Is Hi >= Li for some i?

    which is equivalent to:
    Is Sum(1 / j) for j = 1 to i + 1 >= 100?

    This is a the harmonic series (which is divergent), so essentially Jack will eventually reach the top of the beanstalk.
    What we can further prove is that he will be successful at or before i = (2 ^ 198)-1 (which is essentially a time in seconds)

    Al of course assuming that Jack is an immortal superhero :D

  2. aaronlau | Profile


    Based on calculation, I seem to arrive at the 17-18 sec mark.

    Rationally, i find it difficult to grasp how it can be faster than 100 seconds.

  3. ahsergio | Profile

    i was unable to visualize correctly the last two puzzles, but lets see about this one.
    As the whole plant stretches, the first meter that Jack climbed will become 2 meters, plus one meter that he can climb every pause. So this one meter will eventually (although it will take a long time) make Jack reach the top, i followed this chart to get to my conclusion:
    plant(m) / jack(m)= proportion

  4. suineg | PUZZLE MASTER | Profile

    I dont think so, my answer to this one is NO, because any second it passed his ascendent move represent an smaller percentage of the actual grow of the tree, lets put it in a example the first 3 secondss the man climb:
    t=0 plant= 100 mts man= 0 mts
    t=1 plant 100mts man= 1 mts (the plant grows again)
    plant= 200mts man= 2 mts (because 2/200 is ths same as 1/100 and the man moves with the plant as it grows) now he is at 1% of the size of the plant
    t=2 plant= 200 mts man= 3 mts (the plant grows again)
    plant= 300 mts man= 4.5 mts (same reason as in t=1)
    now the man is at 1.5% os the plant size
    so this pattern continues indefinetely and the man advance a distance that represent smaller and smaller percentage oof the plant size so it will take infiniity to reach so it is impossible I guess jaja I have been wrong before.
    This problem is somehow similar to a numeric calculus problem of a turtle that advance half of the distance it has left to reach the goal every second, nice problem bilbao, my guess is that you teach math in the University for sure, cool.

  5. Sebastian Roughley | Profile

    I think he would. The clue is in the fact that every part of the beanstalk grows at the same time. So Jack would travel during the growth spurts, effectively negating those effects.

  6. abcbcdcdef | Profile

    Yes, he can!Ans can be proved mathematically.

    height he reached after n sec:(n=0 when beanst.=100) n=/=1
    {([([(1×2/1)+1]x3/2)+1]x4/3)+….)x n/n-1)+1}
    =(2/1×3/2×4/3x…xn/n-1) +(3/2×4/3x…xn/n-1)+…+n/n-1 +1
    =n{1 +1/2 + 1/3 +1/4 +…+1/n-1} +1

    The height of the beanstalk=100n

    He will reach the top if
    taking t=0 when beanstalk=0m

    time taken=(1.51×10^43 )+1s; to be exact

    Hopefully my calculation is right…

    Still, he would have to be alive for that long; which, in a fairy tale, is possible.

  7. Shawn | PUZZLE GRANDMASTER | Profile

    No, for so many reasons:

    The beanstalk would eventually topple under it’s own weight, although if the weight of the beanstalk causes the tip to bend downward close enough to Jack, he might be able to reach it.

    The beanstalk would eventually exit the atmosphere and cease to grow.

    The temperature would quickly become too cold for Jack to survive.

    The atmosphere would quickly become to oxygen-poor for Jack to survive.

    Jack would quickly tire and be unable to keep up his grueling pace of 1m/s vertical climb. Climb 3 feet, rest for 1 second, repeat – no thanks!

  8. Bobo The Bear | PUZZLE MASTER | Profile

    If Jack (and the universe around him) had an infinite lifespan, infinite energy reserves, infinite patience, etc. … then yes, he would eventually reach the top.

    After two seconds, Jack has climbed 1 out of 100 meters, or 1% of the way. During the next growth spurt, that percentage stays unchanged. When Jack climbs again, he gains 1 out of 200 meters, or (1/2)% of the remaining distance. The third time he climbs, he gains 1 out of 300 meters, or (1/3)% of the remaining distance, and so on.

    After 4 seconds, he’s gone (1 + 1/2)% of the total distance.
    After 6 seconds, he’s gone (1 + 1/2 + 1/3)%.
    After 8 seconds, he’s gone (1 + 1/2 + 1/3 + 1/4)%.
    After 10 seconds, he’s gone (1 + 1/2 + 1/3 + 1/4 + 1/5)%.
    And so on.

    Mathematically speaking, these fractions form the harmonic series, which diverges – meaning that there’s no limit to how large the sum of the fractions can be. Eventually these fractions will add up to 100% of the distance he needs to go.

    But it will take quite a while. If my calculations are correct, it would take 9.565 * 10^35 years. Considering that the Big Bang model estimates the age of the universe to be 1.373 * 10^13 years, I wouldn’t wait up for him.

  9. TonyTKL | Profile

    Yes, assuming he doesn’t grow old and get tired, that is. ;-) If he’s climbing 1 metre each time, there will be a certain amount of plant growing beneath him. Therefore, there will be a little less growing above him each time.

  10. Mashplum | PUZZLE MASTER | Profile

    Yes. I haven’t figured out how long it will take though.

  11. Falwan | Profile

    Yes, Jack will reach the top after a long time.

  12. jmart | Profile

    No, the ratio that he gains on the plant will drop too small and he’ll actually start to lose ground on the top, limit of 0 as the number of growths goes to infinity.

  13. kasabubu | Profile

    Without doing the math, I’d say “no”, but he will come very, very close to it as the curve (if you’d pictured hes ascend in a graph of a function) will approach the curve of the beanstalk’s growth infinitely.

  14. IrishFutbol | Profile

    No, while the ratio of amount climbed/total length will initially go up at the beginning, after a few seconds it will start to slowly decline to 1%.

  15. Satya | Profile

    Not Possible mathematically, Jack rate of assent is far less than the growth of bean. Even though one may think jack is in advantage with the elastic growth of beanstalk, it is a wrong perception as Jack’s elevation from ground will increase but distance from the top end of stalk will not be reduced.

    I calculated to a negative result :).

    How ever as most pointed out practically beanstalk cease to grow and Jack will tire soon.

  16. suineg | PUZZLE MASTER | Profile

    ok, now I agree with everyone that said that under the inmortal hero condition it could be possible;
    because mathematically is a sum of fraction that has to equal a constant that is 100 percent
    sum (1/i) from i = 1 to ? = 100 and ? is the close to the exact value of seconds it would take, close to a discrete infinity , cool man was tricky a fairy tale jajaja bilbao you are definetely a mathematician, got me but cool.

  17. hex | PUZZLE MASTER | Profile

    The 100 comes from the following:
    Li = 100 * (i + 1)
    Hi = (i + 1) * Sum(1 / j) for j = 1 to i + 1

    Hi >= Li
    (i + 1) * Sum(1 / j) for j = 1 to i + 1 >= 100 * (i + 1)
    Sum(1 / j) for j = 1 to i + 1 >= 100

    More interesting physics facts:
    – As the beanstalk grows, the earth rotation will slow down
    – Eventually, the earth will rotate with the beanstalk
    – Once the beanstalk becomes heavy enough (supposing almost infinite rigidity), it may fall through earth and emerge from the other side of the globe

    The usual pat on the back :D :
    Thanks Bilbao
    Thanks RK

  18. suineg | PUZZLE MASTER | Profile

    cool Hex that is advance math for me to grasp right away jajaja, what I mean with my last comment was the ratio of percentage that represent 1 mts that the man advance in each time the plant grows; first 1% of 100,the plant grows the man climb 1 mts the plant grows again 4.5 of 300 that is 3/2%=(1+1/2)% of 300 so in my case was a much simpler sum, that is the sum of the percentage alone that always sum up to 100, this is somehow very well explained by Bobo The Bear, that sum has to add in some time 100%, the number of terms in that series is like the number of seconds the man take to reach to the top, but this problem is hard and I learn what a divergent harmonic series was that, so thanks, there were very cool explanations, congrats.

  19. RogHyde | Profile

    I don’t understand the maths but can’t see how he can possibly gain on the beanstalk:
    Time Jack Tree Percentage Jack’s
    Climbs Grows of Growth Total
    1 sec 0m 100m 0 0
    2 sec 1m 100m 50% 1 + (1m x 50%)
    3 sec 1m 100m 33.3% 3.5m + (3.5m x 33.3%)
    4 sec 1m 100m 25% 4.66m + (4.6m x 25%)
    etc etc


  20. bilbao | Profile

    Although it seems counterintuitive, the answer to this puzzle is YES. Jack would eventually reach the top of the beanstalk (in our fairy tale context). Most of you got it right!!!
    Some really good math and non math explanations above by Hex, abcbcdcdef and Bobo The Bear.

  21. Mashplum | PUZZLE MASTER | Profile

    I wrote a computer simulation of this problem in order to arrive at the solution. It ran continuously for days without success, but I was still pretty certain that Jack could eventually get there. Thank you math people for proving it, and for proving that the human brain is still far better than a computer. Shame on me for not digging out my old Calculus book in the first place.

  22. hex | PUZZLE MASTER | Profile

    A quick exercise (pushing figures up a little bit):

    Computer CPU speed 5 GHz quad core (ie 4 CPUs in the chip)

    Assume 10 iterations per clock cycle (too exagerated considering current pipelining and execution optimizations especially that the main execution is a division)

    Assume 1.51×10^43 seconds and hence iterations (according to abcbcdcdef)

    Time required to compute:
    1.51×10^43 iterations / (10 iterations per clock cycle) / (5e9 clock cycle per second) / (4 cores per PC)
    = 7.55 x 10^31 seconds
    ~= 2.4 x 10^24 years

    Life is plain too short :(

  23. cocoawatwat | Profile

    Well really i would say yes or no, i mean what would jack really expect to be at the top? In the story there is a giant that wanted to get him. If i were jack i wouldn’t keep climbing. He was poor too so he should work or something instead of climb an endless bean stalk. Also he could just try to chop it down.

  24. Mashplum | PUZZLE MASTER | Profile

    Thanks hex. Here is the program:

    10 let n=1
    20 let j=0
    30 let b=100*n
    40 if j>=b then goto 80
    50 let n=n+1
    60 let j=(j+1)*n/(n-1)
    70 goto 30
    80 print n”, “b”, “int(j)
    90 end

    Do you think after 2 septillion years I would have at least got the right answer with this?

  25. Mashplum | PUZZLE MASTER | Profile

    Of course I would have to multiply n by 2 to get the answer in seconds.

  26. hex | PUZZLE MASTER | Profile

    in Basic? Multiply my time estimate by about 100-1000 depending on whether it is interpreted or compiled (i guess interpreted).

    Why do you have to multiply by 2?

    Anyway, a faster and cleaner implementation would be:
    10 s=0
    20 i=0
    30 i=i+1
    40 s=s+1/i
    50 if s<100 then goto 30
    60 print i-1;”,”;s
    70 end

  27. Mashplum | PUZZLE MASTER | Profile

    It is in Basic because I learned to program in 1988 (after which my forward-looking high school cancelled all its programming classes.)

    I have to multiply by 2 since each loop takes 2 seconds (1 for the beanstalk to grow and 1 for Jack to climb). The same would be true for your program.

  28. hex | PUZZLE MASTER | Profile

    Basic was such a popular language then. I guess it is still widely used (VB.NET)

    Each second Jack climbs up 1 m, and the beanstalk stretches instantaneously. So no doubling needed here.

  29. Puzzler | Profile

    He would never reach the top because when the tree expanded, he would be ripped in half :dead , or he would fall.

  30. james turner | Profile

    Though this is an old problem, it is of interest that one can generate a simple differential equation for the solution. This process is as follows:
    b denotes bean height
    j denotes height Jack has climbed
    vb denote bean rate of growth
    vj denote Jack climb rate
    dj height Jack climbs in each unit of time
    d denotes time increment

    j(t+d) = j(t)*b(t+d)/b(t) + dj

    b(t+d) = b(t) + vb*d (growth rate for beanstalk)

    => j(t+d) – j(t) = j(t)*vb*d/b(t) + vj*d

    Divide by d:

    j(t+d) – j(t) = j(t)*vb/b(t) + vj
    Old Friend the derivative of j(t):

    djdt = j(t)*vb/(b0+vb*t) + vj

    Solve differential equation for j(t):

    j(t) = (b0+vb*t)*log(b0+vb*t)*vj/vb + C)

    Initial condition: @t=1, j(1)=1, b0=100, vj=1, vb = 100

    C = -(2*log(200)-1)/200

    Set b(t) = j(t) to find time Jack reaches the top:

    b0+vb*t = j(t) = ( b0+vb*t)*log(b0+vb*t)*vj/vb + C)

    Solving for T yields,

    t = ( exp( (vb/vj)*(1-c) ) – b0 )/vb

    Plugging in Numbers:

    t = 2*exp(199/2) – 1 = 3.2608509268211d43 Indeed a big number!!!

  31. ladybug48363 | Profile

    Does the beanstalk stretch thinner as it grows?

  32. soumen023 | Profile

    50 seconds he will take

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