If 2 1/2 dozen donuts costs $8.00 how much would 1 dozen cost. Using equivalent ratios.

Question

This is a seventh grade math question from a worksheet on Unit Rates with Fraction. The instructions on the worksheet are not helping to explain and I've had no luck online. The entire problem is; Bill's mom is bringing donuts to school. She has a coupon to get 2 1/2 donuts for $8.00. How much would just one dozen donuts cost at this price. Let c represent the cost of the donuts. 

Answer 1

set up a proportion

2.5/$8.00=1/c where c=cost of 1 dozen

cross multiply

2.5c=8

25c=80

c=80/25

c=3.2

c=$3.20