Ответ:
Пошаговое объяснение:
28 1/7 - 19 2/7 + y = 21 4/7
27 8/7 - 19 2/7 + y = 21 4/7
8 6/7 + y = 21 4/7
y = 21 4/7 - 8 6/7
y = 20 11/7 - 8 6/7
y = 12 5/7
=========или=======
28 1/7 - 19 2/7 + y = 21 4/7
28 1/7 + y = 21 4/7 + 19 2/7
28 1/7 + y = 40 6/7
y = 40 6/7 - 28 1/7
y = 12 5/7