Let, the weight of Grapefruit = x
It would be: x + 1 5/8 + 2 3/16 = 7 1/2
x + 13/8 + 35/16 = 15/2
16x+26+35 /16 = 15/2
16x+61/16 = 15/2
2(16x+61) = 15*16
32x + 122 = 240
x = 118/32
x = 3.68
x = 3 11/18 Pound
In short, Your Answer would be 3 11/18 Pound
Hope this helps!