2015 TJC P2 Q6

Find the number of ways to arrange the twelve letters of the word PERSEVERANCE

1. in a row, [1]

Four letters are selected at random from the twelve letters of the word PERSEVERANCE to form a code word. The code word can contain at most two identical letters. Find the number of possible code words. [3]

TGM interpretation for the four-letter code: allow four distinct letters or exactly one pair of identical letters together with two other distinct letters. Two repeated pairs, such as EERR, are excluded. This makes explicit the convention used by the retained solution.