2/5 + 3/10 = 4/10 + 3/10 = 7/10
7/8 + 5/12 = 42/48 + 20/48 = 62/48 = 31/24
4/7 + 7/9 = 36/63 + 49/63 = 85/63
3/8 + 5/6 = 18/48 + 40/48 = 58/48 = 29/24

Ответ:
7/10, 31/24, 85/63, 29/24