Need asap i don't understand this ?

2-digit numbers less than 91 which are 1 less than a multiple of 10

In triangle abc segment de is parallel to the side ac . (the endpoints of segment de lie on the sides ab and bc respectively). find de, if ac=20cm, ab=17cm, and bd=11.9cm;

Ben bowled 124 and 188 in his first two games. what must he bowl in his third game to have an average of at least 160?