Question

Ellen paid 11.80 for 40 postage stamps. If some were 33 cents and some were 23 cents how many stamps were 33 cents stamps did she buy ?

Answer 1

.23x +.33y =11.80
x+y=40

Use substitution:
x=40-y
.23(40-y) +.33y = 11.80
9.2 - .23y + .33y = 11.80
9.2 + .1y = 11.80
.1y = 2.6
y = 26
Therefore, there are 26 stamps that were 33 cents.