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?

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:
 
 Resolving the system graphically we have:
 
 Note: see attached image for the graphic solution.
 
 234 phones were sold at the wrong price