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 place.

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 A

Explanation:

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]

$$\begin{aligned} \text {Speed upstream =}\\ (\frac{15}{3\frac{3}{4}}) km/hr \\ = 15 \times \frac{4}{15} = 4 km/hr \\ \text{Speed Downstream = } (\frac{5}{2\frac{1}{2}}) km/hr \\ = 5 \times \frac{2}{5} = 2 km/hr \\ \text {So speed of current = } \frac{1}{2}(4-2) \\ = 1 km/hr \end{aligned}$$

Answer: Option 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

Answer: Option C

Explanation:

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

Answer: Option A

Explanation:

$$\begin{aligned} \text{Speed Upstream} = \frac{3}{\frac{20}{60}} = 9 km/hr \\ \text{Speed Downstream} = \frac{3}{\frac{18}{60}} = 10 km/hr \\ \text{Rate of current will be} \\ \frac{10-9}{2} = \frac{1}{2} km/hr \end{aligned}$$