A3 lb bag of one brand of cat food costs $3.52. a 6 lb bag of another brand costs $6.80. which brand is cheaper per pound?

For this case what we should do is find the price per pound of each of the brands of cat food.
 We have then:
 Mark 1: 
 3 lb costs $ 3.52
 P = (3.52) / (3) = 1.17 $ / lb
 Mark 2:
 6 lb costs $ 6.80
 P = (6.80) / (6) = 1.13 $ / lb
  
 Mark 2 is cheaper per pound: 
 1.13 $ / lb