х-(16 8/21 - 5) = 10 1/3
x - 11 8/21 = 10 1/3
x = 10 1/3 + 11 8/21
x = (10+11)+(1/3+8/21) = 21 + (3*7+8/21) = 21 15/21 = 21 5/7
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проверка
21 5/7 -(16 8/21 - 5) = 10 1/3
21 5/7 - 11 8/21 = 10 1/3
(21-11)+(5/7 - 8/21) = 10 +( 5*3-8/21)= 10 + 7/21 = 10 7/21 = 10 1/3
верно!