A motorist travels to place 150 km away at an average speed of 50 km/hr and returns at 30 km/hr. Find…

For equal distances at speeds $x$ and $y$, the average speed is $\frac{2xy}{x+y}=\frac{2\times50\times30}{80}=37.5$ km/hr. Check: 300 km in $3+5=8$ hours gives 37.5 km/hr. The simple average of the two speeds, 40 km/hr, is wrong because more time is spent at the slower speed.