Question

Rajesh travels 20% of his journey by car with a speed of 40km/h, 50% of his journey at a speed of 20 km/ hr and remaining distance with a speed of 30 km/h. What will be the average speed of Rajesh for the whole journey? 
1. 20km/h
2. 30km/h
3. 25km/h
4. 40km/h
5. 42 km/h

Answer 1

Correct Answer - Option 3 : 25km/h

Given:

Rajesh travels 20% of his journey by car with a speed = 40km/h

Rajesh travels 50% of his journey by car with a speed = 20km/h

Rajesh travels 30% of his journey by car with a speed = 30km/h

Formula used:

Average speed = 1/(P/x + Q/y + R/z + ……)

Calculation:

P = 20% = 1/5

⇒ Q = 50% = 1/2

⇒ R = 30% = 3/10

⇒ x = 40km/h

⇒ y = 20km/h

⇒ z = 30km/h

Required average speed = 1/(P/x + Q/y + R/z)

⇒ 1/(1/(5 × 40) + 1/(2 × 20) + 3/(10 × 30))

⇒ 1/(1/200 + 1/40 + 1/100)

⇒ 1/(8/200)

∴ Required average speed is 25km/h