A boat can travel with a speed of 13km/h in still water. If the speed of the stream is 4km/h. Find the time taken by the boat to go 81 km upstream?
Show Hint
Always check the units in the options carefully. 9 hours is equivalent to 540 minutes ($9 \times 60$), which makes Option (B) correct over Option (C) (which says 9 *minutes*).