А)1,25: x=1/4×2 1/2 б)3,4×х=10-1 1/2

1.25 : х = 0.25 * 2                 (1\4 = 0.25)
1.25 : х = 0.5
х = 1.25 : 0.5
х = 2.5

проверка:
1.25 : 2.25 = 0.5
0.25 * 2 = 0.5
0.5 = 0.5

3.4 * х = 10 - 1.5       (= 1.5)
3.4 * х = 8.5
х = 8.5 : 3.4
х = 2.5

проверка
3.4 * 2.5 = 8.5
10 - 1.5 = 8.5
8.5 = 8.5