Problem
Pick a friendly amount for A so both fractions divide evenly; 20 cups works.
Trying a concrete easy number turns an abstract 'how many times' question into plain arithmetic a fifth grader can do.
5.NF.B.6Solve An Easier Related ProblemPicking a convenient amount for A is safe, because the answer asked for is a comparison and not an amount.
Every relation in the problem says one container holds some fraction of another, so making A larger or smaller stretches B and C by the very same factor.
A comparison between two amounts does not move when both are stretched alike, so the answer cannot depend on which starting amount was chosen.
Scaling both sides of a comparison by the same number is multiplying it by that number over itself, which is one.
B is A plus one-fifth of A. One-fifth of 20 is 4, so B is 20 + 4 = 24 cups.
A fraction of a quantity is the whole times the fraction, so one-fifth of 20 is just 20 divided into 5 equal parts.
5.NF.B.4Identify SubproblemsC is three-fourths of B. Three-fourths of 24 is 24 divided by 4 (which is 6) times 3, giving 18 cups.
Three-fourths of an amount means split it into 4 equal parts and take 3 of them.
5.NF.B.4Identify SubproblemsC is 18 cups and A is 20, so C is 18/20 = 9/10 of A.
Because the answer is a ratio, the specific easy number I chose cancels out and the result 9/10 holds for any starting amount.
5.NF.B.6Solve An Easier Related ProblemTake a fraction of an amount by multiplying, do it step by step, and the comparison 9/10 stays the same no matter how much A starts with.