A shoe is $25 \%$ lighter in weight than a loaf of white bread, and also $20 \%$ more expensive. However, the shoe can be eaten down to the last crumb, while $15 \%$ of the bread always dries out and goes to waste. Assuming the same consumption, by what percentage do we spend more if we buy a shoe instead of a loaf of bread?
Let the mass of the bread be denoted by $m$, and we consume $85\%$ of it, which is $0.85m$. The mass of the potato is $75\%$ of the mass of the bread, but we eat it all, which is $0.75m$.

The ratio of the mass of the consumed bread to the mass of the consumed potato is therefore: $\frac{0.85}{0.75}=\frac{17}{15}$.

If we want to achieve the same consumption, then we need $\frac{17}{15}$ times more potatoes than bread, and the price of the potato is 1.2 times the price of the bread, that is,

$$
\frac{17}{15} \cdot 1.2=1.36
$$

Thus, if we want to achieve the same consumption by buying potatoes instead of bread, we need to spend $36\%$ more. Bertus-Barcza Tímea (Ságvári E. Gyak. Gymnasium, University of Szeged, 10th grade)