A person can row a distance of \(3\frac{3}{4}\) km upstream in \(1\frac{1}{2}\) hours and a distance of 13 km downstream in 2 hours. How much time (in hours) will the person take to row a distance of 90 km in still water?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 20
The speed in still water is half the sum of the downstream and upstream speeds, because the stream adds to one and takes away from the other by the same amount.
Upstream speed \(\displaystyle =3\frac{3}{4}\div1\frac{1}{2}=\frac{15}{4}\times\frac{2}{3}=2.5\) km/hr
Downstream speed \(\displaystyle =\frac{13}{2}=6.5\) km/hr
Speed in still water \(\displaystyle =\frac{6.5+2.5}{2}=4.5\) km/hr
Time for 90 km \(\displaystyle =\frac{90}{4.5}=20\) hr