Question

If Kajal covers a distance at the rate of l km/hr and another equal distance at the rate of m km/hr, then find the average speed.
1. lm km/hr
2. (l + m)/2 km/hr
3. (lm)/2 km/hr
4. 2lm/(l +m) km/hr

Answer 1

Correct Answer - Option 4 : 2lm/(l +m) km/hr

Given:

Kajal speeds are l km/hr and m km/hr respectively.

Formula used: 

Average speed = Total distance travelled/Total time taken

Calculation:

Let the distance be 1 km 

Then, 

Average speed = Total distance travelled/Total time taken

⇒ (1 + 1)/(1/l + 1/m)

⇒ 2/[(l + m)/lm]

⇒ 2lm/(l + m)

∴ The average speed is 2lm/(l + m).