Question

A drawer contains 15¢ stamps and some 18¢. The number of 15¢ stamps is four less than three times the number of 18¢ stamps. The total value of all stamps is $1.29. How much of stamp the stamps are 18¢ are in the drawer

Answer 1

15x + 18y = 129
Substituting x = 3y - 4 into equation 1, it becomes
15(3y - 4) + 18y = 129
45y - 60 + 18y = 129
45y + 18y = 129 + 60
63y = 189
y = 189/63 = 3
substituting y = 3 into x = 3y - 4, it becomes
x = 3×3 - 4 = 9-4 = 5
Therefore,

There are 5 15 cent stamps.

Answer 2

Explanation:

n = number of 0.18 stamps

3n - 4 = number of 0.15 stamps

0.15(3n - 4) + 0.18n = 1.29

0.63n = 1.89

n = 1.89/0.63 = 3

There are 3 of the 0.18 stamps.

3n - 4 = 3(3) - 4 = 5

There are 5 of the 0.15 stamps.