since it is algebra:

let n be no. of boxes
1 box has 8 books
n boxes have 8 x n = 8n books

total no. of books is 8n + 4

given total no. of books is 60

so
8n + 4 = 60
8n + 4 - 4 = 60 - 4
8n + 0 = 56
8n = 56
8n ÷ 8 = 56 ÷ 8
n = 7

n boxes mean 7 boxes

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otherwise, unit method :

nos.
boxes: 1u
books: 1u x 8 = 8u

8u + 4 = 60
1u = 7

7 boxes


similar to algebra

Thanks !