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- Question
The speed of a boat in still water in 15 km/hr and the rate of current is 3 km/hr. The distance travelled downstream in 12 minutes is:
Options- A. 1.2 km
- B. 1.8 km
- C. 2.4 km
- D. 3.6 km
- Correct Answer
- 3.6 km
ExplanationSpeed downstream = (15 + 3) kmph = 18 kmph.Distance travelled = ❨ 18 x 12 ❩km = 3.6 km. 60
Boats and Streams problems
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- 1. A boat covers a certain distance downstream in 1 hour, while it comes back in 1½ hours. If the speed of the stream be 3 kmph, what is the speed of the boat in still water?
Options- A. 12 kmph
- B. 13 kmph
- C. 14 kmph
- D. 15 kmph
- E. None of these Discuss
Correct Answer: 15 kmph
Explanation:
Let the speed of the boat in still water be x kmph. Then,Speed downstream = (x + 3) kmph,
Speed upstream = (x - 3) kmph.
∴ (x + 3) x 1 = (x - 3) x 3 2 ⟹ 2x + 6 = 3x - 9
⟹ x = 15 kmph.
- 2. A boat takes 90 minutes less to travel 36 miles downstream than to travel the same distance upstream. If the speed of the boat in still water is 10 mph, the speed of the stream is:
Options- A. 2 mph
- B. 2.5 mph
- C. 3 mph
- D. 4 mph Discuss
Correct Answer: 2 mph
Explanation:
Let the speed of the stream x mph. Then,Speed downstream = (10 + x) mph,
Speed upstream = (10 - x) mph.
∴ 36 - 36 = 90 (10 - x) (10 + x) 60 ⟹ 72x x 60 = 90 (100 - x2)
⟹ x2 + 48x - 100 = 0
⟹ (x+ 50)(x - 2) = 0
⟹ x = 2 mph.
- 3. A motorboat, whose speed in 15 km/hr in still water goes 30 km downstream and comes back in a total of 4 hours 30 minutes. The speed of the stream (in km/hr) is:
Options- A. 4
- B. 5
- C. 6
- D. 10 Discuss
Correct Answer: 5
Explanation:
Let the speed of the stream be x km/hr. Then,Speed downstream = (15 + x) km/hr,
Speed upstream = (15 - x) km/hr.
∴ 30 + 30 = 4 1 (15 + x) (15 - x) 2 ⟹ 900 = 9 225 - x2 2 ⟹ 9x2 = 225
⟹ x2 = 25
⟹ x = 5 km/hr.
- 4. A boat running downstream covers a distance of 16 km in 2 hours while for covering the same distance upstream, it takes 4 hours. What is the speed of the boat in still water?
Options- A. 4 km/hr
- B. 6 km/hr
- C. 8 km/hr
- D. Data inadequate Discuss
Correct Answer: 6 km/hr
Explanation:
Rate downstream = ❨ 16 ❩kmph = 8 kmph. 2 Rate upstream = ❨ 16 ❩kmph = 4 kmph. 4 ∴ Speed in still water = 1 (8 + 4) kmph = 6 kmph. 2 - 5. A man can row at 5 kmph in still water. If the velocity of current is 1 kmph and it takes him 1 hour to row to a place and come back, how far is the place?
Options- A. 2.4 km
- B. 2.5 km
- C. 3 km
- D. 3.6 km Discuss
Correct Answer: 2.4 km
Explanation:
Speed downstream = (5 + 1) kmph = 6 kmph.Speed upstream = (5 - 1) kmph = 4 kmph.
Let the required distance be x km.
Then, x + x = 1 6 4 ⟹ 2x + 3x = 12
⟹ 5x = 12
⟹ x = 2.4 km.
- 6. A boatman goes 2 km against the current of the stream in 1 hour and goes 1 km along the current in 10 minutes. How long will it take to go 5 km in stationary water?
Options- A. 40 minutes
- B. 1 hour
- C. 1 hr 15 min
- D. 1 hr 30 min Discuss
Correct Answer: 1 hr 15 min
Explanation:
Rate downstream = ❨ 1 x 60 ❩km/hr = 6 km/hr. 10 Rate upstream = 2 km/hr.
Speed in still water = 1 (6 + 2) km/hr = 4 km/hr. 2 ∴ Required time = ❨ 5 ❩hrs = 1 1 hrs = 1 hr 15 min. 4 4 - 7. A man rows to a place 48 km distant and come back in 14 hours. He finds that he can row 4 km with the stream in the same time as 3 km against the stream. The rate of the stream is:
Options- A. 1 km/hr
- B. 1.5 km/hr
- C. 2 km/hr
- D. 2.5 km/hr Discuss
Correct Answer: 1 km/hr
Explanation:
Suppose he move 4 km downstream in x hours. Then,Speed downstream = ❨ 4 ❩ km/hr. x Speed upstream = ❨ 3 ❩ km/hr. x ∴ 48 + 48 = 14 or x = 1 . (4/x) (3/x) 2 So, Speed downstream = 8 km/hr, Speed upstream = 6 km/hr.
Rate of the stream = 1 (8 - 6) km/hr = 1 km/hr. 2 - 8. Speed of a boat in standing water is 9 kmph and the speed of the stream is 1.5 kmph. A man rows to a place at a distance of 105 km and comes back to the starting point. The total time taken by him is:
Options- A. 16 hours
- B. 18 hours
- C. 20 hours
- D. 24 hours Discuss
Correct Answer: 24 hours
Explanation:
Speed upstream = 7.5 kmph.Speed downstream = 10.5 kmph.
∴ Total time taken = ❨ 105 + 105 ❩hours = 24 hours. 7.5 10.5 - 9. In one hour, a boat goes 11 km/hr along the stream and 5 km/hr against the stream. The speed of the boat in still water (in km/hr) is:
Options- A. 3 km/hr
- B. 5 km/hr
- C. 8 km/hr
- D. 9 km/hr Discuss
Correct Answer: 8 km/hr
Explanation:
Speed in still water = 1 (11 + 5) kmph = 8 kmph. 2 - 10. A man takes twice as long to row a distance against the stream as to row the same distance in favour of the stream. The ratio of the speed of the boat (in still water) and the stream is:
Options- A. 2 : 1
- B. 3 : 1
- C. 3 : 2
- D. 4 : 3 Discuss
Correct Answer: 3 : 1
Explanation:
Let man's rate upstream be x kmph.Then, his rate downstream = 2x kmph.
∴ (Speed in still water) : (Speed of stream) = ❨ 2x + x ❩ : ❨ 2x - x ❩ 2 2 = 3x : x 2 2 = 3 : 1.
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