Вычислите: 1 - 5/7 : 10/21.

5/7 : 10/21 = 5/7 * 21/10 = (5*21)/(7*10) = 105/70 = 3/2

1 - 3/2 = 2/2 - 3/2 = -1/2

Ответ: -1/2