So Molly has 16 coins, and we know that 9 of them are dollar coins (Total of $9). We also therefore know that Molly has 7 coins that are not dollar coins (16-9=7). Since the remaining 7 coins are 50 cent pieces, we can multiply 7 by .5 to get how much money Molly has in 50 cent coins ($3.50). To find the total amount of money, we add $3.50 + $9 = $12.50.
Equation:
Variable we Know
N= 16
D= 9
Variables we want to know
$
(DX1) + (.5x(N-D) = $
(9X1) + (.5X(16-9) = $
(9) + (.5 X 7) = $
(9) + (3.5) = $
$ = $12.50
