A person travels from A to B at a speed of 40 kmph and returns by increasing his speed by 50%. What is his average speed for both the trips?

Explanation
  Given:

Speed from A to B = 40 km/h

Speed from B to A = 40 km/h + 50% of 40 km/h = 40 + 20 = 60 km/h

Find:
Average speed for the whole journey (both ways).
Formula for average speed when distance is the same but speeds differ:
Average speed = 2 × Speed 1 × Speed 2 Speed 1 + Speed 2 text{Average speed} = frac{2 times text{Speed}_1 times text{Speed}_2}{text{Speed}_1 + text{Speed}_2} ​ ​
Plug in values:
= 2 × 40 × 60 40 + 60 = 4800 100 = 48  km/h = frac{2 times 40 times 60}{40 + 60} = frac{4800}{100} = 48 text{ km/h} = 2 × 40 × 60 ​ 40 + 60 = 4800 /100 = 48  km/h