Answer: Hello there!
Write the things that we know:
if we define a0 to 18¢ stamps and a1 to the 13¢ stamps; we have:
a0 + a1 = 42
a0*$0.18 + a1*$0.13 = $6.66
now we have two equations and two variables, that we can solve
in the first equation we can isolate one of te variables and replace it in the second, for example let's take a0.
a0 + a1 = 42
a0 = 42 - a1
now let's replace it in the second equation.
a0*$0.18 + a1*$0.13 = $6.66
(42 - a1)*$0.18 + a1*$0.13 = $6.66
now let's solve this for a1.
42*$0.18 + a1($0.13 - $0.18) = $6.66
a1*$0.05 = 42*$0.18 - $6.66 = $0.9
a1 = 0.9/0.05 = 18
Then the man buys 18 stamps of 13¢ , and replacing it in the first equation:
a0 + 18 = 42
a0 = 42 - 18 = 24
We can see that the man buys 24 stamps of 18 cents.
