(10 4/13 - х) - 5 11/13 = 3 8/13
10 4/13 - х = 3 8/13 + 5 11/13
10 4/13 - х = 8 19/13
10 4/13 - х = 9 6/13
х = 10 4/13 - 9 6/13
х = 9 17/13 - 9 6/13
х = 11/13
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Проверка: (10 4/13 - 11/13) - 5 11/13 = 3 8/13
9 6/13 - 5 11/13 = 3 8/13
8 19/13 - 5 11/13 = 3 8/13
3 8/13 = 3 8/13