1/15+4/15+1/18+5/18+2/21+5/21+1/24+7/24=

= (1/15 + 4/15) + (1/18 + 5/18) + (2/21 + 5/21) + (1/24 + 7/24) =
= 5/15 + 6/18 + 7/21 + 8/24 =
1/3 + 1/3 + 1/3 + 1/3 = 4/3 = 1 целая 1/3

Пояснения:
5/15 = 1/3 - сократили на 5
6/18 = 1/3 - сократили на 6
7/21 = 1/3 - сократили на 7
8/24 = 1/3 - сократили на 7