A man bought 42 stamps, some 13¢ and some 18¢. How many of each kind did he buy if the cost was $6.66?

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.