Ответ:

№3

Пошаговое объяснение:

1)

4/20 = 4(:4)/(20:4) = 1/5

4/20 = 1/5

2)

4/18 = (4:2)/(18:2) = 2/9

2/9 = 4/18

3)

39/78 = (39:39)/(78:39) = 1/2

39/78 ≠ 1/3

4)

9/21 = (9:3)/(21:3) = 3/7

3/7 = 9/21