Пошаговое объяснение:

№1

1) 5/8 * 4/15 = 1/2 * 1/3 = 1/6

2) 6 3/4 * 1 11/45 = 27/4 * 56/45 = 3/1 * 14/5 = 42/5 = 8 2/5

3) 11/18 * 36 = 11/1 * 2 = 22/1 = 22

№2

3/8 * 24 = 3/1 * 3 = 9/1 = 9 (мальчиков) - всего в классе

№3

(4 - 14/33 * 1 1/21) * 5 5/8 = 20

1) 14/33 * 1 1/21 = 14/33 * 22/21 = 2/3 * 2/3 = 4/9

2) 4 - 4/9 = 3 9/9 - 4/9 = 3 5/9

3) 3 5/9 * 5 5/8 = 32/9 * 45/8 = 4/1 * 5/1 = 20/1 = 20