Alana bought 3/8 pound of Swiss cheese and 1/4 pound of American cheese. Which pair of fractions cannot be used to find how many pounds of cheese she bought in all.

The 3 over 8 because 1 over 4 is 25 from 100 but 3 over 8 do not tell how many she bought in all. there is no number multiply by 8 then gives as 100,? * 8=100 but there is a number multiply by 4 then gives as 100 25 * 4=100 the answer of the question is '3 over 8'