$35$ 44-cents stamps. $5$ 28-cents stamps.
Work Step by Step
Let $x$ be the number of 44 cents (0.44 dollars) stamps. Let $y$ be the number of 28 cents (0.28 dollars) stamps. Sarah and Keith Robinson purchased 40 stamps, a mixture of 44¢ and 28¢ stamps. $x+y=40$ They spent $16.80$. $0.44x+0.28y=16.80$ $x+y=40$ $0.44x+0.28y=16.80$ Multiply the second equation by 100. $44x+28y=1680$ $x=40-y$ $44x+28y=1680$ $44(40-y)+28y=1680$ $1760-44y+28y=1680$ $1760-16y=1680$ $-16y=-80$ $y=5$ $x=40-y$ $x=40-(5)$ $x=35$ $35$ 44-cents stamps. $5$ 28-cents stamps.