Пошаговое объяснение:

№3

а) 1/3 + 3/6 = 2/6 + 3/6 = 5/6

b) 4/15 + 1/3 = 4/15 + 5/15 = 9/15 = 3/5

c) 1/2 + 1/12 = 6/12 + 1/12 = 7/12

№4

a) 5/6 + 2/18 = 15/18 + 2/18 = 17/18

b) 7/20 + 4/10 = 7/20 + 8/20 = 15/20 = 3/4

c) 1/4 + 9/16 = 4/16 + 9/16 = 13/16