The probability that a coin is in the [math]i[/math]th box is [math]1/(i+1)[/math]. If you search in the [math]i[/math]th box and it is there, you find it with probability [math]i/(1+i)[/math]. Determine the probability that the coin is in second box, given that you have looked in the fourth box and not found it.

References

Doyle, Peter G. (2006). "Grinstead and Snell's Introduction to Probability" (PDF). Retrieved June 6, 2024.