Riddle:
You forgot a digit of a phone number and make random guesses. What is the probability that you'll guess correctly in at most two attempts? Three attempts?
Python Solution:
Riddle source:
https://archive.org/details/yaglom-yaglom-challenging-mathematical-problems-with-elementary-solutions-vol.-1/page/23/mode/2up?q=forgotten
https://www.cut-the-knot.org/Probability/RecollectingForgottenDigit.shtml