Question 201380
Let x = the number of 15-cent stamps and y = the number of 20-cent stamps.
x($0.15)+y($0.20) = $4.00 and...
x = 3y-8  "The number of 15-cent stamps (x) is eight less than three times the number of 20-cent stamps (3y-8)"  Substitute the x here into the first equation and solve for y.
(3y-8)(0.15)+y(0.20) = $4.00 Simplify.
(0.45)y-1.20+(0.20)y = 4.00
(0.65)y-1.20 = 4.00 Add 1.20 to both sides.
(0.65)y = 5.20 Divide both sides by 0.65
y = 8 and x = 3(8)-8 = 16
There are sixteen 15-cent stamps and eight 20-cent stamps.
Check:
16($0.15)+8($0.20) = $2.40+$1.60 = $4.00 OK!