(12 целых 5/13 +y)-9 целых 9/13=7целых 7/13
(12 5/13 + у) - 9 9/13 = 7 7/13 12 5/13 + у = 7 7/13 + 9 9/13 12 5/13 + у = 16 16/13 у = 16 16/13 - 12 5/13 у = 4 11/13 --------------------- Проверка: (12 5/13 + 4 11/13) - 9 9/13 = 7 7/13 16 16/13 - 9 9/13 = 7 7/13 7 7/13 = 7 7/13