A set of 18 flash cards shows the letters A, B, C, ... , R, one letter per card. Meera turns over one card at random. What is the probability that the letter on it comes after M in the alphabet?

The letters after M and up to R are N, O, $P$, Q and R - five cards out of 18. $P=\frac{5}{18}.\frac{13}{18}$ counts A to M, the letters that do NOT come after M; $\frac{1}{3}$ counts M to R and so wrongly includes M itself; $\frac{1}{2}$ assumes M splits the 18 cards into two halves, but M is the 13th letter, not the 9th.