<PRE><B>Question 956681</B>
X=number of $0.13; Y=number of $0.18
X+Y=42
X=42-Y
$0.13X+$0.18Y=$6.66 Substitute for X.
$0.13(42-Y)+$0.18Y=$6.66
$5.46-$0.13Y+$0.18Y=$6.66 Subtract $5.46 from each side.
$0.05Y=$1.20 Divide each side by $0.05
Y=24 ANSWER1: He bought 24-18¢ stamps. 
X=42-Y=42-24=18 ANSWER 2: He bought 18-13¢ stamps.
CHECK:
$0.13X+$0.18Y=$6.66
$0.13(18)+$0.18(24)=$6.66
$2.34+$4.32=$6.66
$6.66=$6.66</PRE>