dijawab no 12,13,15 pakai cara ya :)

12.
Rumus =
(23 - x) + x + (18 - x) + 4 = 40
41 + 4 - x = 40
45 - x = 40
-x = 40 - 45 = -5
x = 5
13.
31 - 18 + 18 + 22 - 18 = 35
15.
35 -9 + 9 + 25 - 9 + 7 = 58