Let's call the 13¢ stamps a and the 18¢ stamps b:
a+b = 42 and therefore a= 42-b (formula 1)
0.13a+0.18b= 6.66 In this formula, substitute the value of a according to formula 1:
0.13(42-b)+0.18b= 6.66 Multiply on the left to get rid of the parenthesis:
5.46-0.13b+0.18b= 6.66 Subtract 5.46 from both sides:
-0.13b+0.18b= 1.20 Add on the left:
0.05b= 1.20 Divide both sides by 0.05
b= 24 You have 24 18¢ stamps and:
42-24= 18 13¢ stamps
Check: (24 x 0.18) + (18 x 0.13)= 6.66 Correct.
