Решение:

a) 34 - (x - 7) = 18

34 - x + 7 = 18

41 - x = 18

x = 41 - 18

x = 23

б) (x + 24) – 15 = 12

x + 24 - 15 = 12

x + 9 = 12

x = 12 - 9

x = 3

в) (56 - x) + 34 = 72

56 - x + 34 = 72

90 - x = 72

x = 90 - 72

x = 18

г) 62 + (15 + x) = 100

62 + 15 + x = 100

77 + x = 100

x = 100 - 77

x = 23