Speed, Time and Distance
 A man goes uphill with an average speed of 35 km/hr, and covers down with an average speed of 45 km/hr. The distance travelled in both the cases being the same , the average speed for the entire journey is ?

 38 ^{3}/_{8} km/hr.
 39 ^{3}/_{8} km/hr.
 40 km/hr
 None of these

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Average speed = 2uv / (u + v)
Correct Option: B
Average speed = 2uv / (u + v)
= ( 2 x 35 x 45) / (35 + 45) km/hr.
= 39 ^{3}/_{8} km/hr
 If a boy takes as much time in running 10 m as a car takes in covering 25 m, the distance covered by the boy during the time the car covers 1 km, is ?

 400 m.
 40 m.
 250 m.
 650 m.

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Let distance covered by boy is Y.
Then, 25 : 10 : : 1000 : Y
Correct Option: A
Let distance covered by boy is Y.
Then, 25 : 10 : : 1000 : Y
⇒ Y = (10 x 1000)/25 = 400 m
 The ratio between the rates of walking of A and B is 2:3 , if the time taken by B to cover a certain distance is 36 minutes, the time taken by A to cover that much distance is ?

 24 min
 54min
 48 min
 21.6min

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Ratio of speed = 2 : 3
Ratio of times taken for same distance = 1/2 : 1/3
From question 1/2 : 1/3 = t : 36Correct Option: B
Ratio of speed = 2 : 3
Ratio of times taken for same distance = 1/2 : 1/3
From question 1/2 : 1/3 = t : 36
⇒ (1/3) x t = (1/2) x 36
∴ x = 54 min .
 A man, on tour travel first 160 km. at 64 km/hr. and the next 160 km. at 80 km/hr. The average speed for first 320 km. of the tour is ?

 35.55 km/hr
 71.11 km/hr
 36 km/hr
 72 km/hr

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Average speed = 2uv / ( u + v ) km/hr
Correct Option: B
Average speed = 2uv / ( u + v ) km/hr
=(2 x 64 x 80) / (64 + 80) km/hr
= (2 x 64 x 80) / 144 km/hr.
= 71.11 km/hr
 By walking at 3/4 of his usual speed , a man reaches his office 20 minutes later than usual . this usual time is ?

 30 minutes
 60 minutes
 75 minutes
 1 hour 30 minutes

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At 3/4 of the usual speed, the time taken is 4/3 of the usual time
∴ 4/3 of usual time  usual time = 20 minCorrect Option: B
At 3/4 of the usual speed, the time taken is 4/3 of the usual time
∴ 4/3 of usual time  usual time = 20 min
⇒ 4x/3  x = 20
⇒ x/3 = 20
∴ x = 60 min.