The speed of the boat in still water is 20 km/hr and the speed of the stream is 5 km/hr. Find the time taken to travel 50 km downstream and 30 km upstream by the boat.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 4 hours
Going downstream the stream helps the boat, so the speeds add; going upstream the stream works against it, so they subtract.
Downstream speed \(=20+5=25\) km/hr; time for 50 km \(=50\div25=2\) h
Upstream speed \(=20-5=15\) km/hr; time for 30 km \(=30\div15=2\) h
Total time \(=2+2=4\) hours