You divide the Price by the Quantity $$$/quantity to get cost per unit
by the way your previous question for pens should be 89.9 cents or rounded up to 90 cents
(I did the following questions regarding how much you get per dollar, not how much a unit cost)
You are getting about 417 square feet of lawn food per dollar or about 416 square feet of lawn food per dollar. First option is the better option
You are getting about 111 sheets of paper for a dollar or 119 sheets for a dollar. Second option is the better option