a textbook store sold a combined total of 296 sociology and history text books in a week. the number

Let h = history book and s = sociology books. We have 2 equations:

(1) h = s - 42
(2) h + s = 296

Substitute (1) to (2)
s - 42 + s = 296

Combine variables
2s - 42 = 296

Add 42 to each side
2s = 338

Divide each side by 2
s = 169

So h = 169 - 42 = 127

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