Rosemarie took orders for muffins and doughnuts from 18 friends (each person ordered only one item). the doughnuts cost $0.40 and the muffins cost $0.55 each for a total of $9.15. how many muffins did she buy? ( show a step by step process on how to solve)

Total muffins and doughnuts = 18

Assumed all doughnuts = 18 x $0.4 = $7.20

But she paid $9.15.
$9.15 - $7.20 = $1.95 
$1.95 must have been used to pay for the muffin.

Difference in price between muffin and doughnuts:
$0.55 - $0.40 = $0.15

Number of muffins bought = $1.95 ÷ 0.15 = 13
She bough 13 muffins.

Number of doughnut bought = 18 - 13 = 5
She bought 5 doughnuts

She bought 5 doughnuts and 13 muffins.