x*.32 + (40-x) *.23 = 12.08
distribute
.32x +9.20 -.23x = 12.08
combine like terms
.09x +9.20 = 12.08
subtract 9.20
.09x =2.88
divide by .09
x=32
40-32 =8
He bought 32 32cent stamps and 8 23 cent stamps
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