Question 19290
First, take the $3.50 that she will spend on the coffee from the $10.00 that she has to start with.  This leaves ($10.00 - $3.50) $6.50.
Now the question is: How many doughnuts, at $0.40 each, can she buy with $6.50?

Answer: Divide $6.50 by $0.40 = 16.25 doughnuts. But, since she probably can't buy a quarter of a doughnut, then the maximum (whole) doughnuts she can buy is 16.