1. A man can row upstream 10 kmph and downstream 20 kmph. Find the man rate in still water and rate of the stream.
A) 0,5
B) 5,5
C) 15,5
D) 10,5
Ans. C
If a is rate downstream and b is rate upstream
Rate in still water = 1/2(a+b)
Rate of current = 1/2(a-b)
=> Rate in still water = 1/2(20+10) = 15 kmph
= Rate of current = 1/2(20-10) = 5 kmph
2. A man takes 3 hours 45 minutes to row a boat 15 km downstream of a river and 2 hours 30 minutes to cover a distance of 5 km upstream. Find the speed of the current.
A) 1 km/hr
B) 2 km/hr
C) 3 km/hr
D) 4 km/hr
Ans. A
First of all, we know that
speed of current = 1/2(speed downstream – speed upstream) [important]
So we need to calculate speed downstream and speed upstream first.
Speed = Distance / Time [important]
Speed upstream =(15334)km/hr=15×415=4km/hrSpeed Downstream = (5212)km/hr=5×25=2km/hrSo speed of current = 12(4-2)=1km/hr
3. In one hour, a boat goes 11km along the stream and 5 km against it. Find the speed of the boat in still water
A) 6
B) 7
C) 8
D) 9
Ans. C
We know we can calculate it by 1/2(a+b)
= 1/2(11+5) = 1/2(16) = 8 km/hr
4. 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
A) 2 km/hr
B) 3 km/hr
C) 4 km/hr
D) 5 km/hr
Ans. D
Let the speed of the stream be x km/hr. Then,
Speed downstream = (15 + x) km/hr,
Speed upstream = (15 – x) km/hr
So we know from question that it took 4(1/2)hrs to travel back to same point.
So,
3015+x-3015-x=412=>900225-x2=92=>9×2=225=>x=5km/hr
5. If Rahul rows 15 km upstream in 3 hours and 21 km downstream in 3 hours, then the speed of the stream is
A) 5 km/hr
B) 4 km/hr
C) 2 km/hr
D) 1 km/hr
Ans. D
Rate upstream = (15/3) kmph
Rate downstream (21/3) kmph = 7 kmph.
Speed of stream (1/2)(7 – 5)kmph = 1 kmph
