Algebra
posted by Word Problem .
Can someone please help with this?
Allen bought 20 stamps at the post office in 37¢ and 20¢ denominations. If the total cost of the stamps was $7.06, how many 37¢ stamps did Allen buy?
A) 15
B) 16
C) 17
D) 18

Set up a system of equations to model this situation. (Note that a system of equations is TWO different equations.)
Let x be the number of 37cent stamps, and let y be the number of 20cent stamps.
What two equations did you get? 
x+y=7.06

That's only one equation.
Anyway, the number of stamps that he buys is 20.
The other equation will deal with the stamp prices.
What do you get? 
37x+20y=7.06

That's close. The stamps are in CENTS, though.
.37x + .20y = 7.06
That is one equation. The other equation deals with how many stamps he bought. What is it? 
x+y=20
and
37x + .20y = 7.06 
Good. Our two equations are:
.37x + .20y = 7.06
x + y = 20
We're trying to find the number of 37cent stamps (x). We can solve this system of equations by substitution. Solve the second equation for y, and then plug that into the first equation.
We now only have x's in the equation, so solve it as you would any other equation. 
Would the answer be D  18 .37 stamps?

Yup. Nice job.