1. A man can row 913 kmph in still water and finds that it takes him thrice as much time to row up than as to row, down the same distance in the river. The speed of the current is.
A) 323kmph
B) 423kmph
C) 523kmph
D) 623kmph
Ans. B
Speed of current = 1/2(a-b)
Let the speed upstream = x kmph
Then speed downstream is = 3x kmph
speed in still water = 12(a+b)=>12(3x+x)=>2x as per question we know, 2x=913=>2x=283=>x=143
So,
Speed upstream = 14/3 km/hr, Speed downstream 14 km/hr.
Speed of the current
=12[14-143]=143=423kmph
2. A man rows 750 m in 675 seconds against the stream and returns in 7 and half minutes. His rowing speed in still water is
A) 4 kmph
B) 5 kmph
C) 6 kmph
D) 7 kmph
Ans. B
Rate upstream = (750/675) = 10/9 m/sec
Rate downstream (750/450) m/sec = 5/3 m/sec
Rate in still water = (1/2)*[(10/9) + (5/3)] m/sec.
= 25/18 m/sec
= (25/18)*(18/5) kmph
= 5 kmph
3. If a boat goes 7 km upstream in 42 minutes and the speed of the stream is 3 kmph, then the speed of the boat in still water is
A) 12 kmph
B) 13 kmph
C) 14 kmph
D) 15 kmph
Ans. B
Rate upstream = (7/42)*60 kmh = 10 kmph.
Speed of stream = 3 kmph.
Let speed in sttil water is x km/hr
Then, speed upstream = (x ?3) km/hr.
x-3 = 10 or x = 13 kmph
4. 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 stream is
A) 3:1
B) 1:3
C) 2:4
D) 4:2
Ans. A
Let speed downstream = x kmph
Then Speed upstream = 2x kmph
So ratio will be,
(2x+x)/2 : (2x-x)/2
=> 3x/2 : x/2 => 3:1
5. A man’s speed with the current is 20 kmph and speed of the current is 3 kmph. The Man’s speed against the current will be
A) 11 kmph
B) 12 kmph
C) 14 kmph
D) 17 kmph
Ans. C
Speed with current is 20,
speed of the man + It is speed of the current
Speed in still water = 20 – 3 = 17
Now speed against the current will be
speed of the man – speed of the current
= 17 – 3 = 14 kmph
