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.

A. 2 hrs B. 3 hrs C. 5 hrs D. 4 hrs

Answer Answer: Option D Explanation: Speed downstream = (13+4) km/hr = 17 km/hr
Time taken to travel 68 km downstream =(68/17) hrs = 4 hrs

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?

A. 2 mph B. 2.5 mph C. 3 mph D. 4 mph

Answer Answer: Option A Explanation: 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.

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?

A. 2 mph B. 2.5 mph C. 3 mph D. 4 mph

Answer Answer: Option A Explanation: 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.

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?

A. 2.4km B. 2.5 km C. 3 km D. 3.6 km

Answer Answer: Option A Explanation: 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.