A cup is \(\dfrac{1}{3}\) full of milk. What part of the cup is still to be filled to make it full?
\(\dfrac{2}{3}\)
Idea: A completely full cup is one whole (\(1\)). Right now it has only \(\dfrac{1}{3}\) filled. The empty part is what is left to reach \(1\).
Step 1: Write the whole as a fraction with denominator 3.
\(1 = \dfrac{3}{3}\)
Step 2: Subtract the filled part from the whole.
\(\dfrac{3}{3} - \dfrac{1}{3}\)
Step 3: Subtract the numerators (keep the same denominator).
\(\dfrac{3 - 1}{3} = \dfrac{2}{3}\)
Conclusion: The part still to be filled is \(\dfrac{2}{3}\) of the cup.