25/8oz if you use a thrid of it how much is she gone have left over

Lets convert the fraction first to ease the calculations:
2 5/8 = 16/8 + 5/8 = 21/8
so now we can operate easily, to calculate we need to multiply 21/8 by 2/3 that is because, if 1/3 is used then 2/3 are left over (1/3 + 2/3 = 1, the total), so we have:
(2/3)(21/8) = (2*21)/(3*8)
= 42/24
= 21/12
= 7/4
so 7/4 are left over