Chapter 1 - Section 1.5 - Problem Solving and Using Formulas - Exercise Set - Page 67: 30

The two costs will be equal when $10$ rentals are made. The cost of 10 rentals is ${{\$}} 90$ in either store.

Let x = number of rentals Cost at store A = $9x$ Cost at store B = $4x+50$ The two costs will be equal when $9x=4x+50$ $9x-4x=50$ $5x=50$ $x=10$ rentals made. Calculate the cost at A: $9(10)={{\$}} 90$ The two costs will be equal when $10$ rentals are made. The cost of 10 rentals is ${{\$}} 90$ in either store.