Sally had a book collection that she had been working on for quite a while. she was given seventeen books from her 6th grade teacher and now she has 56 books. how many books were in her collection before her teachers donation?

write an equation to represent the problem, and then solve the equation.

Given 17 books.
Total 56 books.
How many books were in her collection before her teachers donation?

First lets write an equation:

b = books before

17 + b = 56

Lets solve for b:

17 + b = 56
Inverse Operation:
56 - 17 = b
56 - 17 = 39
b = 39

Check:- 

17 + b = 56
17 + 39 = ?
17 + 39 = 56
Correct!

There were 39 books in her collection before her teacher donated her books. 

The equation is: 17 + b = 56.
b = 39

Heya dear

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both have same length