2
Q:

# A man swims downstream 72 km and upstream 45 km taking 9 hours each time; what is the speed of the current  ?

 A) 1 kmph B) 3.2 kmph C) 1.5 kmph D) 2 kmph

Explanation:

72 --- 9
? ---- 1

=> Down Stream = 8

45 ---- 9
? ---- 1

=> Up Stream = 5

Speed od current S = ?

S = (8 - 5)/2 = 1.5 kmph.

Q:

A boat running upstream takes 8 hours 48 minutes to cover a certain distance, while it takes 4 hours to cover the same distance running downstream. What is the ratio between the speed of the boat and speed of the water current respectively?

 A) 7:4 B) 11:4 C) 4:7 D) 8:3

Explanation:

Let the speed of the boat upstream be p kmph and that of downstream be q kmph

Time for upstream = 8 hrs 48 min = $\fn_jvn 8\tfrac{4}{5}$ hrs

Time for downstream = 4 hrs

Distance in both the cases is same.

=> p x $\fn_jvn 8\tfrac{4}{5}$ = q x 4

=> 44p/5 = 4q

=> q = 11p/5

Now, the required ratio of Speed of boat : Speed of water current

$\fn_jvn \frac{q+p}{2}:\frac{q-p}{2}$

=> (11p/5 + p)/2  : (11p/5 - p)/2

=> 8 : 3

6 71
Q:

In one hour, a boat goes 12 km/hr along the stream and 6 km/hr against the stream. The speed of the boat in still water (in km/hr) is:

 A) 9 B) 8 C) 7 D) 7.5

Explanation:

Speed in still water = Average of Speed in Upstream and speed in Downstream

= 1/2 (12 + 6) kmph = 9 kmph.

6 37
Q:

A motorboat running upstream takes 8 hours 48 minutes to cover a certain distance, while it takes 4 hours to cover the same distance running downstream. What is the ratio between the speed of the motorboat and speed of the water current respectively?

 A) 3:7 B) 7:9 C) 5:4 D) 8:3

Explanation:

Let the man's rate upstream be x kmph and that downstream be y kmph.

Then, distance covered upstream in 8 hrs 48 min = Distance covered downstream in 4 hrs.

 x x 8 4 = (y x 4) 5
 44 x =4y 5
 y = 11 x. 5
 Required ratio = y + x : y - x 2 2
 = 16x x 1 : 6x x 1 5 2 5 2
 = 8 : 3 5 5

= 8 : 3.

7 33
Q:

A man can row upstream at 16 km/hr and downstream at 24 km/hr. Find the speed of the current.

 A) 4 kmph B) 6 kmph C) 5 kmph D) 3 kmph

Explanation:

Speed of the current = 24-16/2

= 8/2

= 4 km/hr.

6 55
Q:

A motorboat can go 10 miles downstream on a river in 20 min. It takes 30 min for this boat to go back at the same 10 miles. Find the speed of the current ?

 A) 8 m/h B) 5 m/h C) 7 m/h D) 6 m/h

Explanation:

As the distance travelled is constant, the time taken is inversely proportional to speed.

Let 'u' be the speed of the current and 'v' be the speed of the boat.

Speed of the boat downstream = v + u & upstream = v - u

=> v+u/v-u = 30/20 => v/u = 5 => v = 5u

=> v + u = 6u = 10/20/60 miles/hr

=> 6u = 30 m/h

=> u = 5 m/h