21.  A boat can travel with a speed of 13 km/hr in still water.If the speed of the stream is 4 km/hr find the time taken by the boat to go 68 km downstream.


2 hrs
3 hrs
5 hrs
4 hrs


Answer

 Option

Speed downstream = (13+4) km/hr = 17 km/hr Time taken to travel 68 km downstream =(68/17) hrs = 4 hrs

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22.  The speed of a boat in still water is 10 km/hr. If it can travel 26 km downstream and 14 km upstream in the same time, the speed of the stream is?


2 km/hr
2.5 km/hr
3 km/hr
4 km/hr


Answer

 Option

Let the speed of the stream be x km/hr. then, speed downstream = (10+x) km/hr, speed upstream = (10-x) km/hr. therefore 26/(10+x) = 14/(10-x) <=> 260 - 26x =140+ 14x <=> 40x = 120 <=> x=3 km/hr.

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23.  A boat takes 90 minutes less to travel 36 miles downstream then to travel the same distance upstream. If the speed of the boat in still water is 10 km/hr, the speed of the stream is?


2 mph
2.5 mph
3 mph
4 mph


Answer

 Option

Let the speed of the stream be x mph. then, speed downstream = (10+x) mph, speed upstream = (10-x) mph. therefore 36/(10-x) - 36/(10+x) = 90/60 <=> 72*60= 90 (100-x^2) <=> x^2 +48x+100=0 <=>(x+50) (x-2) =0 <=> x=2 mph.

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24.  A boat takes 90 minutes less to travel 36 miles downstream then to travel the same distance upstream. If the speed of the boat in still water is 10 km/hr, the speed of the stream is?


2 mph
2.5 mph
3 mph
4 mph


Answer

 Option

Let the speed of the stream be x mph. then, speed downstream = (10+x) mph, speed upstream = (10-x) mph. therefore 36/(10-x) - 36/(10+x) = 90/60 <=> 72*60= 90 (100-x^2) <=> x^2 +48x+100=0 <=>(x+50) (x-2) =0 <=> x=2 mph.

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25.  A man can row at 5 km/hr in still water. If the velocity of current is 1 km/hr and it takes him 1 hour to row to a place and come back, how far is the place?


2.4km
2.5 km
3 km
3.6 km


Answer

 Option

Speed downstream = (5+1) km/hr = 6 km/hr ; speed upstream = (5-1) km/hr = 4 km/hr. Let the required distance be x km. Then, x/6 + x/4 = 1 <=> 2x+3x =12 <=> 5x =12 <=> x = 2.4 km.

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