There are two possible ways to solve this; ill add them both, and whichever way is convenient to you, JUST USE IT!
1st way: (I consider this way)
Change the mixed numbers into improper fractions.
7⅞ - 3¼ = 63/8 - 13/4
give the fractions a common denominator
63/8 - (2)*13/4*(2) = 63/8 - 26/8
= 37/8
change the improper fraction to a mixed number
37/8 = 4⅝
2nd way:
I like to write the numbers above each other:
7 7/8
3 1/4
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For the fractions, you need to have the same denominator (8), so convert 1/4 to 2/8.
7 7/8
3 (2)*1/4*(2) ⇒ 3 2/8
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Now subtract the fractions and the whole numbers separately. Fortunately there is no need to borrow, so this step is very easy:
7 7/8
-3 2/8
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4 5/8
Answer: 4 5/8