What equation do I use

Question

Mr. Prime bought new art supplies for his class. Markers cost $2.20 per box and colored pencils cost $1.40 per box. Mr. Prime bought a total of 18 boxes and spent $35.60. How many boxes of each art supply did he buy?

Answer 1

m + p = 18

2.2m + 1.4p = 35.6

1.4m + 1.4p = 1.4(18) = 25.2

subtract to eliminate one variable

.8m= 10.4

m= 10.4/.8 = 52/4 = 13 marker boxes, 5 pencil boxes

check the answers

2.2(13)+ 1.4(5)

=28.6+ 7

= 35.6