Aman bought 42 stamps, some 13¢ and some 18¢. how many of each kind did he buy if the cost was $6.66? if x represents the number of 13 cent stamps and y the 18 cent stamps, which system represents the problem?

Solution:

Here we have 2 system:
1. x + y = 42
2. 13x + 18y = 666

From equation 1.:
3. y = 42 - x

Substitute equation 3 into equation 2.:
13x + 18(42 - x) = 666
-5x + 756 = 666
5x = 90
x = 18

Since we got the value of x, then we will find the value of y. Substitute x into Equation 3:
y = 42 - 18 = 24