13. Вычислите: 9/10 + 4/9 : (2 - 1 11/21) - 1 2/3

2 - 1 11/21 = 2 - 32/21 = 42/21 - 32/21 = 10/21.
4/9 : 10/21 = 4/9 * 21/10 = (4*21)/(9*10) = 84/90 = 14/15.
9/10 + 14/15 - 1 2/3 = 27/30 + 28/30 - 50/30 = (27+28-50)/30 = 5/30 = 1/6.