1. The problem statement, all variables and given/known data There is a 1 in 4 chance that on any given sexual encounter, Joe will contract an STD. If Joe has sex 4 times, what are the chances of him having an STD? 2. Relevant equations Something with combinations maybe? 3. The attempt at a solution I thought you would just add up the probability of the event happening for each trial, but that would give me 100% which isn't right.