Erasers cost $5 per carton and pencils cost $7 per carton. If an order comes in for a total of 15 cartons for $85, how many cartons of each were bought?

Answer 1

5x+7y=85
substitute y for x: 5(15-y)+7y=85
75-5y+7y=85
75+2y=85
2y=10
y=5
So 5 pencils @ $7=$35 plus10 erasers @ $5, or $50, total $85.

Answer 2

Easier way:
$5*3=$15
$7*10=$70
70+15=$85
3 cartons of erasers were bought,and 10 cartons of pencils were bought