8a + 15 = 55
8a = 55 - 15
8a = 40 / : 8
a = 5
30 - 5b = 20
-5b = 20 - 30
-5b = -10 / : (-5)
b = 2
4c - 12 = 16
4c = 16 + 12
4c = 28 / : 4
c = 7
(10 - x) : 4 = 1 / · 4
10 - x = 4
-x = 4 - 10
-x = -6 / · (-1)
x = 6
18 : (y - 8) = 9 / · (y - 8)
18 = 9(y - 8)
9y - 72 = 18
9y = 18 + 72
9y = 90 / : 9
y = 10
(3d + 9) : 7 = 3 / · 7
3d + 9 = 21
3d = 21 - 9
3d = 12 / : 3
d = 4