How many 15 cent stamps are in the drawer?

Question

a drawer contains 15 cent stamps and 18 cent stamps. The number of 15 cent stamps is four less than three times the number of 18 cent stamps. The total value of all the stamps is $1.29. How many 15 cent stamps are in the drawer?

Answer 1

step 1. convert the total value into pennies, so $1.29 = 129 cents

 

step 2. let x = number of 18 cent stamps

 

step 3. then, the number of 15 cent stamps will be 3x - 4

 

step 4. our equation will be 18x + 15(3x - 4) = 129

 

step 5. expanding, we get 18x + 45x - 60 = 129

 

step 6. combining, we get 63x = 189

 

step 7. solving, x = 3 then number of 18 cent stamps

 

step 8. then 3x - 4 = 9 - 4 = 5 the number of 15 cent stamps

 

Proof: (18 x 3) + (15 x 5) = 54 + 75 = 129