16Hit1600
AlgebraHardFixed fee plus unit price, then a new total

Algebra practice question

An online bookshop charges a fixed delivery fee plus the same price for each book. An order of 3 books costs $41.50 and an order of 7 books costs $85.50. What is the cost, in dollars, of an order of 10 books?

Answer: 118.5

4 extra books cost $44, so each book is $11. Then the fee is 41.50 − 33 = 8.50. Ten books: 110 + 8.50 = 118.50.

Common wrong answers

If you answered 110
110 leaves out the delivery fee.
If you answered 138.33
138.33 scales $41.50 by 10/3, treating the fee as per-book.
If you answered 122.14
122.14 scales $85.50 by 10/7.

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