Sahil can row 3 km against the stream in 20 minutes and he can return in 18 minutes. What is rate of current ?

Answer: Option 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

Answer: Option D

Explanation:

Rate upstream = (15/3) kmph
Rate downstream (21/3) kmph = 7 kmph.
Speed of stream (1/2)(7 - 5)kmph = 1 kmph

Answer: Option C

Explanation:

We know we can calculate it by 1/2(a+b)

=> 1/2(11+5) = 1/2(16) = 8 km/hr

Answer: Option 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

Answer: Option 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 $$\begin{aligned} =\frac{1}{2}[14 - \frac{14}{3}]\\ = \frac{14}{3} = 4 \frac{2}{3} kmph \end{aligned}$$