Abby can buy an 8 pound bag of dog food for $7.40 or a 4 pound bag of the same dog food for $5.38 which is the better buy

The 8 pound bag is better.
Divide the price by the weight, and you get the price per pound, which you can compare directly. 
8 pound: 7.40/8 = $0.93 per pound
4 pound: 5.38/4 = $1.35 per pound

Yeah what she said right

Lil