The random ant question is asked in this post. I reproduce it below for completeness.

Question: $500$ ants are randomly put on a 1-foot string (independent uniform distribution for each ant between 0 and 1). Each ant randomly moves toward on end of the string (equal probability to the left or the right) at constant speed of 1 foot/minute until it falls of a t one end of the string. Also assume that the size of the ant is infinitely small. When two ants collide head-on, they both immediately change directions and keep on moving at 1 foot/min. What is the expected time for all ants to fall off the string?

The question above is equivalent to asking the expected value of the maximum of $500$ IID random variables with uniform distribution between $0$ and $1$.

We know that the expected value of $\max(X_1,...,X_{500})$ where $X_1,...,X_{500}$ are IID, is $\frac{500}{501}$, as shown in another post.

However, the answer given to the random ant question is $\frac{499}{500},$ which I fail to decipher.

  • 1
    $\begingroup$ On a rather unrelated tangent that may be interesting or completely coincidental, a problem on the silver level USACO (USA Computing Olympiad) competition of December 2019 had a very similar problem if the solution to the problem could be of any help: usaco.org/index.php?page=viewproblem2&cpid=967 $\endgroup$ – Soham Konar Jan 1 at 3:11
  • $\begingroup$ physics.montana.edu/avorontsov/teaching/problemoftheweek/… is also concluding $N/(N+1)$ as mentioned mean time when initially there were $N$ ants. $\endgroup$ – dan_fulea Jan 1 at 3:27
  • $\begingroup$ Could you please point out where exactly the answer $\frac{499}{500}$ is given? I could find it neither in the post you linked to, nor in the one that it links to. $\endgroup$ – joriki Jan 1 at 4:20
  • $\begingroup$ Actually the answer that I posted above comes from 'A practical guide to quantitative finance interview'. The book has exactly the same question above and the answer that it gives is $\frac{499}{500}.$ $\endgroup$ – Idonknow Jan 1 at 4:27

The guide that gives $\frac{499}{500}$ as an answer is wrong. You're right that it should be $\frac{500}{501}$.

| cite | improve this answer | |
  • $\begingroup$ What if the ants were moving at a different speed. Instead of $1 foot/min$, what if they were moving at $x foot/min$? Would the answer simply be $\frac{500}{501}x$? $\endgroup$ – David May 6 at 13:31
  • $\begingroup$ Also, what if the ants don't start out with equal probability of moving to the right or left. What if there was bias towards one direction? Based on symmetry, it seems that this would not affect the answer? $\endgroup$ – David May 6 at 13:34
  • $\begingroup$ Can't edit my first comment anymore, but I think it's wrong. It should be $\frac{\frac{500}{501}}{x}$ $\endgroup$ – David May 6 at 13:38
  • $\begingroup$ @David: Yes to both your questions (with the correction). $\endgroup$ – joriki May 6 at 14:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.