The first thing you should do is calculate how many pounds of cheese a hamburger has
1 hamburger = 1/4 pound of cheese.
Then, you can make a rule of three to know how many pounds of cheese were sold:
1 ---> 1/4
310 ---> x
Clearing x we have:
x = ((310) / (1)) * (1/4) = 77.5
Answer
they sell last saturday 77.5 pounds of cheese