Imagine an election between two candidates.
A receives m votes, B receives n votes, and A wins (m>n).
If the ballots are cast one at a time, what is the probability that A will lead all the way throughout the voting process?
(In reply to reference
Excellent reference, showing several ways to prove it.
It is however a challenge to show that (m-n)/(m+n) is a valid answer for m>=n : without going "copy and paste".