Solve this problem by using a two order system. A man bought 42 stamps, some 13¢ and some 18¢. How many of each kind did he buy if the cost was $6.66? a0 18¢ stamps and a1 13¢ stamps

0.13x+0.18 (42-x) = 6.66

Solve for x

X=18 for 0.13

42-18=24 for 0.18

Given a0 18¢ stamps and a1 13¢ stamps

a0 + a1 = 42

and

0.18a0 + 0.13a1 = 6.66

multiply first eqn by 0.13

0.13a0 + 0.13a1 = 42*0.13 = 5.46

subtract that from second eqn

0.18a0 - 0.13a0 = 6.66 - 5.46

0.05a0 = 1.20

a0 = 24

a1 = 42 - a0 = 42 - 24

a1 = 18