#### Answer

$35$ 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.