The simplest way to do this is to get it into improper fraction form with the same denominator.
First things first, get it into an improper fraction.
7 7/8 = 7*8+7/8 3 1/4 = 3*4+1/4
= 56+7/8 = 12+1/4
= 63/8 = 13/4
The next thing to do is to get the denominators the same by finding the LCF, which could be done by dividing the 63/8 by 4, which would give a decimal as a numerator, so its best to multiply 13/4 by two
The two fraction will then be:
63/8 and 26/8
Seeing as the denominators are the same, you can simply subtract the numerators and simplify, i.e.
63-26/8 = 37/8
= 4 5/8
Hope this makes sense :D
