Answer
The cost of the biology book is 91 dollars.
Work Step by Step
Let X represent the cost of a used textbook for college algebra.
Twice the cost of a used textbook for college algebra means 2X.
15 more than twice the cost of a used textbook for college algebra means 2X + 15.
Since the cost of the two books together is 129, we can set up the following equation: 2X + 15 + X = 129.
3X + 15 = 129
Subtract 15 from both sides
3X = 114
Divide both sides by 3
X = 38
Since the cost of the biology book is 2X + 15, and X = 38, the cost of the biology book is 91 dollars.
