Question Detail
In one hour, a boat goes 11km along the stream and 5 km against it. Find the speed of the boat in still water
- 6
- 7
- 8
- 9
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
1. In one hour, a boat goes 11km along the stream and 5 km against it. Find the speed of the boat in still water
- 6
- 7
- 8
- 9
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
2. 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
- 2 km/hr
- 3 km/hr
- 4 km/hr
- 5 km/hr
Answer: Option 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}
3. Sahil can row 3 km against the stream in 20 minutes and he can return in 18 minutes. What is rate of current ?
- 1/2 km/hr
- 1/3 km/hr
- 2 km/hr
- 4 km/hr
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}
4. 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.
- .4 km
- 1.4 km
- 2.4 km
- 3.4 km
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
5. 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.
- 1 km/hr
- 2 km/hr
- 3 km/hr
- 4 km/hr
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}