First you can choose from 26 letters, then from 25 (because you can't use the same letter) then from 24, and then from 23. You multiply 26 * 25 * 24 * 23 and you get your answer.
To solve you first need to pick out the most important information, ~Spent 20 minutes ~Each prob takes 2 minutes Lets have y be the total time spent, and x the number of probs (y/2) = x Hope this helped!! ^-^
