3 1/4 + n = 7 1/8
subtract 3 1/4 from both sides
n = 7 1/8 - 3 1/4
make that 1/4 into 2/8
n = 7 1/8 - 3 2/8
make both numbers into improper fractions
on the left it's 7 * 8 + 1
on the right it's 3 * 8 + 2
n = 57/8 - 26/8
since they have the same bottoms, you can put them together as one fraction
n = (57 - 26)/8
n = 31/8
if you want, you can turn it back into a mixed fraction
n = 3 7/8