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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
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