## 255. Math Puzzle $8 / 86$

Felix rides his bicycle on a straight route between two towns that are ten kilometers apart, and the round trip takes him one hour.

The next day, a strong wind blows, so he rides 5 km/h faster on the way there and 5 km/h slower on the way back compared to his average speed from the previous day.

How much time does he save, under otherwise identical conditions?
1. Day, at constant speed: $v=\frac{s}{t}=\frac{20}{1}=20 \mathrm{~km} / \mathrm{h}$

2. Day,

Outbound trip: $t_{H}=\frac{s}{v_{H}}=\frac{10}{25}=\frac{2}{5} \mathrm{~h}=24 \mathrm{~min}$

Return trip: $t_{R}=\frac{s}{v_{H}}=\frac{10}{15}=\frac{2}{3} \mathrm{~h}=40 \mathrm{~min}$

$t_{H}+t_{R}=64 \mathrm{~min}$

The trip on the second day took 4 minutes longer.