Chapter 3.10, Problem 18C

Interestingly, this technique can also be used to solve problems that could be solved by writing a single equation, like the next one.

While traveling on business, Eldrick bought a $50 prepaid international cell phone to call his girlfriend Lindsey. Calls cost $0.07 per minute. How many minutes has he used if the display shows $32.15 remaining?

To answer this question we could think about two different equations. One states that the value of the card V is $32.15.

Next we can write another equation that states that the value V started at $50 and decreased by $0.07 for each minute used x.

Now solve the system using the substitution method. Did you get the same solution? Use this result to solve the original problem.