Speed, Time and Distance
 A train travelled at a speed of 35 km/hr for the first 10 minutes and at a speed of 20 km/hr for the next 5 minutes. The average speed of the train for the total 15 minutes is

 30 km/hr
 23 km/hr
 31 km/hr
 29 km/hr

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Distance covered = 35 × 10 + 20 × 5 km 60 60 = 35 + 10 km 6 6 = 45 km 6
Total time = 15 minutes= 1 hour 4 ∴ Required average speed = Distance covered Time taken = 45 × 4 6
= 30 kmphCorrect Option: A
Distance covered = 35 × 10 + 20 × 5 km 60 60 = 35 + 10 km 6 6 = 45 km 6
Total time = 15 minutes= 1 hour 4 ∴ Required average speed = Distance covered Time taken = 45 × 4 6
= 30 kmph
 A bus covers four successive 3 km stretches at speed of 10 km/hr, 20 km/hr, 30 km/hr and 60 km/hr respectively. Its average speed over this distance is

 30 km/hr
 25 km/hr
 20 km/hr
 10 km/hr

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Average speed = Total distance Time taken = 12 3 + 3 + 3 + 3 10 20 30 60
=12 3 6 + 3 + 2 + 1 60 = 12 × 60 = 20 kmph 3 × 12 Correct Option: C
Average speed = Total distance Time taken = 12 3 + 3 + 3 + 3 10 20 30 60
=12 3 6 + 3 + 2 + 1 60 = 12 × 60 = 20 kmph 3 × 12
 With an average speed of 40 km/hr, a train reaches its destination in time. If it goes with an average speed of 35 km/hr, it is late by 15 minutes. The total journey is

 30 km
 40 km
 70 km
 80 km

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Let the length of journey be x km, then
x − x = 15 = 1 35 40 60 4 ⇒ 8x − 7x = 1 280 4 ⇒ x = 280 = 70 km 4 Correct Option: C
Let the length of journey be x km, then
x − x = 15 = 1 35 40 60 4 ⇒ 8x − 7x = 1 280 4 ⇒ x = 280 = 70 km 4
 P travels for 6 hours at the rate of 5 km/ hour and for 3 hours at the rate of 6 km/ hour. The average speed of the journey in km/ hour is


3 1 5 
5 1 3 
1 2 9 
2 2 5


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Total distance
= 5 × 6 + 3 × 6
= 30 + 18 = 48 km
Total time = 9 hours∴ Average speed = 48 = 16 = 5 1 kmph 9 3 3 Correct Option: B
Total distance
= 5 × 6 + 3 × 6
= 30 + 18 = 48 km
Total time = 9 hours∴ Average speed = 48 = 16 = 5 1 kmph 9 3 3
 A man covers the journey from a station A to station B at a uniform speed of 36 km/hr and
returns to A with a uniform speed of 45 km/hr. His average speed for the whole journey is :

 40 km/hr
 40.5 km/hr
 41 km/hr
 42 km/hr

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Here same distances are covered at different speeds.
∴ Average speed = 2xy kmph x + y = 2 × 36 × 45 kmph (36 + 45) = 2 × 36 × 45 = 40 kmph 81 Correct Option: A
Here same distances are covered at different speeds.
∴ Average speed = 2xy kmph x + y = 2 × 36 × 45 kmph (36 + 45) = 2 × 36 × 45 = 40 kmph 81