A textbook store sold a combined total of 307 biology and chemistry textbooks in a week. The number

A textbook store sold a combined total of 307 biology and chemistry textbooks in a week. The number of chemistry textbooks sold was 71 less than the number of biology textbooks sold. How many textbooks of each type were sold?

Let b be the number of biology books and c be the number of chemistry books. We have two equations:

1. b + c = 307
2. c = b - 71

Substitute (2) into (1) for c
b + (b - 71) = 307

Combine like terms:
2b - 71 = 307

Using our equation solver, we get:
b = 189

Now substitute that into (2):
c = 189 - 71
c = 118