Hi I am after some help starting this question.

Question

A local hardware store sells three types of boxes; small, medium and large. The cost of the boxes are; small
$10, medium $20 and large $30. In the previous week a total of 1200 boxes were sold resulting in proceeds
of $20 000 for the week. The number of small boxes sold was equal to the sum of the medium and large
boxes sold.
Use matrix methods to determine how many of each type of box was sold.