Решение:

a) 40 + 20 = 60, 60 : 20 = 3, 3 + 13 = 16

б) 50 + 40 = 90, 90 : 3 = 30, 30 - 9 = 21

в) 100 - 70 = 30, 30 - 8 = 22, 22 + 28 = 50, 50 : 20 = 2,5

г) 90 - 60 = 30, 30 : 15 = 2, 2 + 14 = 16, 16 * 5 = 80