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

Answer 1

Using system of equations:
E=number of cartons of erasers
P=number of cartons of pencils
5E+7P=85
E+P=15
Solve for E and P.

Easier: We know E+P=15, so directly write
5E+7(15-E)=85
Solve for E and then find P=15-E.

Even easier, do it mentally:
15 cartons of erasers cost 5*15=75.
Since total cost is $85, so the $10 left is to exchange erasers for pencils at $(7-5)=$2 per carton, which means there are 5 cartons of pencils, and 15-5=10 cartons of erasers.