Find the probability of a 4 turning up at least once in two tosses of a fair die.

Two tosses of a fair die give 6 × 6 = 36 equally likely outcomes.

It is easier to count the opposite: no 4 in either toss. Each toss then has 5 good outcomes, so 5 × 5 = 25 outcomes.

P(at least one 4) = 1 − 25/36 = 11/36.

5/36 is a tempting wrong choice: it is the count of one fixed pattern, not the full set of cases.