Speed, Time and Distance
- The speed of a motorboat is 30 km/h. It starts from Mumbai at 6 am and reaches Goa at 8 am. Same day, it starts from Goa at 12 pm and reaches Mumbai at 4 pm. Find the the speed of stream.
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Clearly, motorboat takes 2 h go from Mumbai to Goa and 4 h to go from Goa to Mumbai. i.e.,
Time taken to row upstream = 2 x Time taken to row downstream
∴ n = 2
Let the speed of stream be V km/h.
According to the formula,
Speed of boat = V(n + 1)/(n - 1)Correct Option: C
Clearly, motorboat takes 2 h go from Mumbai to Goa and 4 h to go from Goa to Mumbai. i.e.,
Time taken to row upstream = 2 x Time taken to row downstream
∴ n = 2
Let the speed of stream be V km/h.
According to the formula,
Speed of boat = V(n + 1)/(n - 1)
⇒ 30 = V(2 + 1)/(2 - 1)
∴ V = 30/3 = 10 km/h
- The current of a stream runs at 1 km/h. A motorboat goes 35 km upstream and back again at the starting point in 12 h. What is the speed of motorboat in still water ?
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Let the speed of boat in still water be V km/h. Then,
35/(V - 1) + 35/(V + 1) = 12
⇒ 35(V + 1 + V - 1) = 12(V2 - 1)Correct Option: C
Let the speed of boat in still water be V km/h. Then,
35/(V - 1) + 35/(V + 1) = 12
⇒ 35(V + 1 + V - 1) = 12(V2 - 1)
⇒ 6V2 - 35V - 6 = 0
⇒ (V - 6)(6V + 1) = 0
⇒ V = 6 (V = - 1/6 is not possible)
So, speed of boat in still water is 6 km/h.
- A streamer goes downstream from one port to another in 4 h. It covers the same distance upstream in 5 h. If the speed of the stream is 2 km/h , find the distance between the two ports ?
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Let the distance between the two ports = L km
Then, speed downstream = L/4
and speed upstream = L/5
∴ Speed of the stream = 1/2(L/4 - L/5)Correct Option: D
Let the distance between the two ports = L km
Then, speed downstream = L/4
and speed upstream = L/5
∴ Speed of the stream = 1/2(L/4 - L/5)
⇒ 1/2[(5L - 4L)/20] = 2
⇒ L = 80 km
- A person can row 71/2 km/h in still water. It takes him twice as long to row up a distance as to row down the same distance. Find the speed of the stream ?
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Speed upstream + Speed downstream = 2 x 71/2 = 15 km/h
Since, the time taken is in the ratio 2 : 1
hence the speed will be in the ratio 1 : 2
∴ Speed upstream = (1/3) x 15 = 5 km/h
and speed downstream = (2/3) x 15 = 10 km/hCorrect Option: C
Speed upstream + Speed downstream = 2 x 71/2 = 15 km/h
Since, the time taken is in the ratio 2 : 1
hence the speed will be in the ratio 1 : 2
∴ Speed upstream = (1/3) x 15 = 5 km/h
and speed downstream = (2/3) x 15 = 10 km/h
∴ Speed of stream = (10 - 5)/2 = 2.5 km/h
- A thief running at 8 km/h is chased by a policeman whose speed is 10 km/h. If the thief is 100 m ahead of the policeman, then the time required for the policeman to catch the thief will be
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Relative speed of policeman with thief = (10 - 8) = 2 km/h
Now, time taken to cover 100 m with speed of 2 km/h or 2000 m/h
Now, Speed = Distance / Time
∴ Time = D/S = 100/2000Correct Option: B
Relative speed of policeman with thief = (10 - 8) = 2 km/h
Now, time taken to cover 100 m with speed of 2 km/h or 2000 m/h
Now, Speed = Distance / Time
∴ Time = D/S = 100/2000
= 1/20 h or 60/20 min = 3 min