9/13 + 2/13 + z = 15/13
z = 15/13 - (9/13 + 2/13)
z = 15/13 - 11/13
z = 4/13
Проверка: 9/13 + 2/13 + 4/13 = 15/13
9/13 + 2/13 + z = 15/13
z = 15/13 - (9/13 + 2/13)
z = 15/13 - 11/13
z = 4/13
Проверка: 9/13 + 2/13 + 4/13 = 15/13