Решение:

1) 3/8 = х/40

8х = 3•40

х = (3•40)/8

х = (3•5)/1

х = 15.

2) 5/6 = 15/х

5х = 6•15

х = (6•15)/5

х = (6•3)/1

х = 18.

3) 56/72 = х/9

7/9 = х/9

х = 7.