Jason has a total of 43 stamps, and we have two different stamps involved.
x = 15 cents
y = 20 cents
We will need to set up two equations and then use substitution:
43 = x + y (represents total stamps)
7.50 = 0.15x + 0.20y (represents total value)
To use substitution, we'll use the first equation and get either x or y on its own. In this case I will choose to get y on its own:
43 - x = x - x + y (subtract x to get y alone)
43 - x = y
Now we will use this in the second equation and simplify:
7.50 = 0.15x + 0.20(43 - x)
7.50 = 0.15x + 8.6 - 0.20x
7.50 = 8.6 - 0.05x
-1.1 = -0.05x (divide by -1.1 to get x alone)
22 = x
Now that we know Jason has 22 of the stamps that are worth 15 cents, we need to find y by plugging x, 22, into the first equation:
43 = 22 + y
21 = y
Jason has 22 stamps that are worth 15 cents and 21 stamps that are worth 20 cents.