Question

An error occurred in your bookkeeping department this month. The price of one of the smart phones that you sell is $489. Several customers were charged $565 for the phone instead. You know that the total sales for this month were $213,873 and that 401 phones were sold. How many phones were sold at the wrong price?

Answer 1

The first thing we must do for this case is to define a variable.
 We have then:
 x: number of phones that were sold at the wrong price.
 y: number of phones that were sold with the correct price.
 We now write the system of equations that represents the problem.
 We have then:
 $565x + 489y = 213873 x + y = 401$
 Resolving the system graphically we have:
 $x = 234 y = 167$
 Note: see attached image for the graphic solution.
 Answer:
 234 phones were sold at the wrong price