Question 211654
X = 18-cent stamps 
2x = 8-cent stamps
X+3 = 13-cent stamps 
You had the .08 cent & .13 cent stamps mixed up.
.18x+.08*2x+.13(x+3)=3.68
.18x+.16x+.13x+.39=3.68
.47x=3.68-.39
.47x=3.29
x=3.29/47
x=7 18 cent stamps.
2*7=14 8 cent stamps
7+3=10 13 cent stamps.
Proof:
.18*7+.08*14+.13*10=3.68
1.26+1.12+1.30=3.68
3.68=3.68