3
Q:

# A boat travels 72 km downstream in 8 hours and 84 km upstream in 12 hours. Find the speed of the boat in still water and the speed of the water current ?

 A) 9 and 3 kmph B) 6 and 7 kmph C) 8 and 1 kmph D) 7 and 2 kmph

Answer:   C) 8 and 1 kmph

Explanation:

Downstream speed = 72km/8hrs  = 9 kmph
upstream speed = 84km/12hrs = 7 kmph
speed of boat = avg of downstream and upstream speeds
speed of boat = (9+7)/2kmph = 8 kmph.
current speed = half of the difference of downstream and upstream speeds
currend speed = (9-7)/2kmph = 1 kmph

Q:

A boy can swim in still water at 4.5 km/h, but takes twice as long to swim upstream than downstream. The speed of the stream is ?

 A) 1.8 kmph B) 2 kmph C) 2.2 kmph D) 1.5 kmph

Explanation:

Speed of Boy is B = 4.5 kmph

Let the speed of the stream is S = x kmph

Then speed in Down Stream = 4.5 + x

speed in Up Stream = 4.5 - x

As the distance is same,

=> 4.5 + x = (4.5 - x)2

=> 4.5 + x = 9 -2x

3x = 4.5

x = 1.5 kmph

6 1283
Q:

The current of a stream at 1 kmph. A motor boat goes 35 km upstream and back to the starting point in 12 hours. The speed of the motor boat in still water is ?

 A) 8 kmph B) 6 kmph C) 7.5 kmph D) 5.5 kmph

Explanation:

Speed of the stream = 1
Motor boat speed in still water be = x kmph
Down Stream = x + 1 kmph
Up Stream = x - 1 kmph
[35/(x + 1)] + [35/(x - 1)] = 12
x = 6 kmph

5 1313
Q:

A boatman can row 96 km downstream in 8 hr. If the speed of the current is 4 km/hr, then find in what time he will be able to cover 8 km upstream ?

 A) 1.5 hrs B) 1 hrs C) 2.5 hrs D) 2 hrs

Explanation:

Speed in downstream = 96/8 = 12 kmph

Speed of current = 4 km/hr

Speed of the boatman in still water = 12 – 4 = 8 kmph

Speed in upstream = 8 – 4 = 4 kmph

Time taken to cover 8 km upstream = 8/4 = 2 hours.

3 1288
Q:

A man rows his boat 60 km downstream and 30 km upstream taking 3 hrs each time. Find the speed of the stream ?

 A) 5 kmph B) 10 kmph C) 15 kmph D) 45 kmph

Explanation:

Speed of the boat downstream  $\inline \fn_jvn \small s=\frac{d}{t}$ = $\inline \fn_jvn \small \frac{60}{3} = 20 kmph$

Speed of the boat upstream $\inline \fn_jvn \small s= \frac{d}{t} = \frac{30}{3} = 10 kmph$

$\fn_jvn&space;\small&space;\therefore$ The speed of the stream = $\inline \fn_jvn \small \frac{(speed of downstream - speed of upstream)}{2} = 5kmph$.

8 1684
Q:

A man goes down stream with a boat to some destination and returns upstream to his original place in 5 hours. If the speed of the boat in still water and the strean are 10km/hr and 14km/hr respectively, the distance of the destination from the string place is

 A) 16 km B) 18 km C) 21 km D) 25 km

Explanation:

Let the distance covered be D km.

$\inline&space;\frac{D}{10+4}+\frac{D}{10-4}=5$

$\Rightarrow&space;\frac{D}{14}+\frac{D}{6}=5$

$\inline&space;\Rightarrow&space;10D=&space;42\times&space;5=210$

$\inline&space;\Rightarrow&space;D=&space;21$  km