Translate the following inference to logic notation: For all x, if x_20 writes a letter, then x_20 is a cashier. William is a cashier if and only if Susan is listening to a song. William is writing a letter if and only if Linda is playing tennis. Therefore if Linda plays tennis, then Susan is listening to a song.
forall x: P(x) -> Q(x). Q(a) <-> p. P(a) <-> q. Therefore q -> p.