Walter packed $\frac{4}{5}$ kg of flour into as many bags of $\frac{1}{4}$ kg as possible and had some flour left. What was the mass of the flour left?

1. Find the maximum number of full bags that can be packed.

Number of bags $= \frac{4}{5} \div \frac{1}{4} = \frac{4}{5} \times 4 = \frac{16}{5} = 3\frac{1}{5}$
Since the number of bags must be a whole number, Walter can pack $3$ full bags.

2. Calculate the mass of flour packed into the 3 full bags.

Mass of flour packed $= 3 \times \frac{1}{4} = \frac{3}{4}$ kg

3. Find the mass of the flour left.

Mass of flour left $= \frac{4}{5} - \frac{3}{4} = \frac{16}{20} - \frac{15}{20} = \frac{1}{20}$ kg