Объяснение:
б) по св-ву пропорции:
12(5-z) = 8(18-5z)
3(5-z) = 2(18-5z)
15-3z = 36 -10z
7z = 21
z = 3
в) по св-ву пропорции:
7(15х-3) = 3(7-35х)
7(5x-1) = 7-35x
35x-7 = 7-35x
5x-1 = 1-5x
10x = 2
x = 0.2
