Question 192256
let the number of 20 cent and 28 cent stamps by x and y respectively
:
x+y=140........eq 1
.2x+.28y=31.2..eq 2
:
rewrite eq 1 to x=140-y and plug it into eq 2
:
.2(140-y)+.28y=31.2
:
28-.2y+.28y=31.2
:
.08y=3.2
:
y=40 (28 cent stamps)
:
x=140-y=140-40=100 (20 cent stamps)