7.  The speed of a boat in still water in 15 km/hr and the rate of current is 3 km/hr. The distance travelled downstream in 12 minutes is:  
Answer: Option D Explanation:
Speed downstream = (15 + 3) kmph = 18 kmph.

8.  A boat takes 90 minutes less to travel 36 miles downstream than to travel the same distance upstream. If the speed of the boat in still water is 10 mph, the speed of the stream is:  
Answer: Option A Explanation:
Let the speed of the stream x mph. Then, Speed downstream = (10 + x) mph, Speed upstream = (10 – x) mph.
72x x 60 = 90 (100 – x^{2}) x^{2} + 48x – 100 = 0 (x+ 50)(x – 2) = 0 x = 2 mph. 
9.  A man can row at 5 kmph in still water. If the velocity of current is 1 kmph and it takes him 1 hour to row to a place and come back, how far is the place?  
Answer: Option A Explanation:
Speed downstream = (5 + 1) kmph = 6 kmph. Speed upstream = (5 – 1) kmph = 4 kmph. Let the required distance be x km.
2x + 3x = 12 5x = 12 x = 2.4 km. 
10.  A boat covers a certain distance downstream in 1 hour, while it comes back in 1 hours. If the speed of the stream be 3 kmph, what is the speed of the boat in still water?  
Answer: Option D Explanation:
Let the speed of the boat in still water be x kmph. Then, Speed downstream = (x + 3) kmph, Speed upstream = (x – 3) kmph.
2x + 6 = 3x – 9 x = 15 kmph. 