stuck with stamps....

[terbear14]

ok, so i've tried this over and over again and cannot figure out how to solve for "C"

A stamp collection has 11 cent stamps, 15 cent stamps and 21 cent stamps. There are twice as many 11 cent stamps as there are 15 cent stamps. There is a total of 30 stamps with a value of $4.22. How many 21 cent stamps are there?

So I got to:

11 cent stamps = A
15 cent stamps = B
21 cent stamps = C


11 cent stamps = 2B

then i get stuck!

Please help!

 

[rbcc]

Hi, so since you have 2 unknowns you are going to need 3 equations in order to find a unique solution to each variable

you got

A=2B

but you also need

There is a total of 30 stamps

A+B+C=30

with a value of $4.22

0.11A+0.15B+0.21C=4.22

do you know how to solve from there?