A man travels the first half of a journey at $30$ km/hr and the second half at $60$ km/hr. Find his average speed for the whole journey.

For equal distances, average speed is the harmonic mean: $\dfrac{2xy}{x+y} = \dfrac{2 \times 30 \times 60}{90} = \dfrac{3600}{90} = 40$ km/hr. Not the arithmetic mean $(30 + 60)/2 = 45$ — that holds for equal *times*, not equal *distances*.