Unit rate is how much is 1 pound cost. So you are taking how much pounds and dividing that number by that number. If you did that to the top of the proportion you also have to do that to the bottom to keep the ratio the same. The unit rate is important because you can determined the best buy. Also if you know what the unit rate is you can figure out how much 55 pounds cost.
Item One Proportion
Item Two Proportion
The better buy is the 1st one that has 7lb for $28.98 because the unit rate is 1lb for $4.14 The other buy is 3lb for $13.98. The unit rate for the second one is $4.66. The 1st choice is cheaper buy $0.52.