# Chemistry Questions Answers

11) A man rows 750 m in 675 seconds against the stream and returns in 7 and half minutes. His rowing speed in still water

- 1) 4 kmph
- 2) 5 kmph
- 3) 6 kmph
- 4) 7 kmph

**Ans.**B

**Explanation :**

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

12) 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)

- 1) 2 km/hr
- 2) 3 km/hr
- 3) 4 km/hr
- 4) 5 km/hr

**Ans.**D

**Explanation :**

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, \begin{aligned} \frac{30}{15+x} - \frac{30}{15-x} = 4\frac{1}{2} \=> \frac{900}{225 - x^2} = \frac{9}{2} \=> 9x^2 = 225 \=> x = 5 km/hr \end{aligned}

13) A man can row \begin{aligned} 9\frac{1}{3} \end{aligned} 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 i

- 1) \begin{aligned} 3\frac{2}{3}kmph \end{aligned}
- 2) \begin{aligned} 4\frac{2}{3}kmph \end{aligned}
- 3) \begin{aligned} 5\frac{2}{3}kmph \end{aligned}
- 4) \begin{aligned} 6\frac{2}{3}kmph \end{aligned}

**Ans.**B

**Explanation :**

Friends first we should analyse quickly that what we need to calculate and what values we require to get it. So here we need to get speed of current, for that we will need speed downstream and speed upstream, because we know Speed of current = 1/2(a-b) [important] Let the speed upstream = x kmph Then speed downstream is = 3x kmph [as per question] \begin{aligned} \text{speed in still water = } \frac{1}{2}(a+b) \=> \frac{1}{2}(3x+x) \=> 2x \\text{ as per question we know, }\2x = 9\frac{1}{3} \=> 2x = \frac{28}{3} => x = \frac{14}{3} \\end{aligned} So, Speed upstream = 14/3 km/hr, Speed downstream 14 km/hr. Speed of the current \begin{aligned} =\frac{1}{2}[14 - \frac{14}{3}]\= \frac{14}{3} = 4 \frac{2}{3} kmph \end{aligned}

14) 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

- 1) 3:1
- 2) 1:3
- 3) 2:4
- 4) 4:2

**Ans.**A

**Explanation :**

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

15) 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

- 1) 12 kmph
- 2) 13 kmph
- 3) 14 kmph
- 4) 15 kmph

**Ans.**B

**Explanation :**

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 â€”3) km/hr. x-3 = 10 or x = 13 kmph

16) 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

- 1) 11 kmph
- 2) 12 kmph
- 3) 14 kmph
- 4) 17 kmph

**Ans.**C

**Explanation :**

If you solved this question yourself, then trust me you have a all very clear with the basics of this chapter. If not then lets solve this together. 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

17) A boat can travel with a speed of 16 km/hr in still water. If the rate of stream is 5 km/hr, then find the time taken by the boat to cover distance of 84 km downstre

- 1) 4 hours
- 2) 5 hours
- 3) 6 hours
- 4) 7 hours

**Ans.**A

**Explanation :**

It is very important to check, if the boat speed given is in still water or with water or against water. Because if we neglect it we will not reach on right answer. I just mentioned here because mostly mistakes in this chapter are of this kind only. Lets see the question now. Speed downstream = (16 + 5) = 21 kmph Time = distance/speed = 84/21 = 4 hours

18) A man can row at 5 kmph in still water. If the velocity of the current is 1 kmph and it takes him 1 hour to row to a place and come back. how far is that pla

- 1) .4 km
- 2) 1.4 km
- 3) 2.4 km
- 4) 3.4 km

**Ans.**C

**Explanation :**

Let the distance is x km Rate downstream = 5 + 1 = 6 kmph Rate upstream = 5 - 1 = 4 kmph then x/6 + x/4 = 1 [because distance/speed = time] => 2x + 3x = 12 => x = 12/5 = 2.4 km

19) The speed of a boat in still water is 15 km/hr and the rate of current is 3 km/hr. The distance travelled downstream in 12 minutes

- 1) 1.6 km
- 2) 2 km
- 3) 3.6 km
- 4) 4 km

**Ans.**C

**Explanation :**

Speed downstreams =(15 + 3)kmph = 18 kmph. Distance travelled = (18 x 12/60)km = 3.6km

20) 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

- 1) 12 kmph
- 2) 13 kmph
- 3) 14 kmph
- 4) 15 kmph

**Ans.**B

**Explanation :**

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 â€”3) km/hr. x-3 = 10 or x = 13 kmph