You multiply 3/4 by 3/3 to get the dominator of 12 and get 9/12 then 1/3 by 4/4 to get the denominator of 12 and get 4/12 . Now that you have a common denominators you simplify 4/12 to 1/3 bc you divided 4/12 by 4 bc 4 is the greatest you can divide from. So Rob needs 1/3 cups of butter