Answer:
There are sixteen 15-cent stamps and eight 20-cent stamps.
Explanation:
Stamp collector has:
15c stamps
20c stamps
Total value is $4
x($0.15) + y($0.20) = $4.00
The # of 15c stamps is 8 - than 3 * the # of 20c stamps; 15c = 8 - 3c
x = 3y - 8
(3y-8)(0.15)+y(0.20) = $4.00
0.45)y-1.20+(0.20)y = 4.00
(0.65)y-1.20 = 4.00
(0.65)y = 5.20
y = 8
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