The ratio of Rani's swimming speed in still water to the speed of the river is 7 : 1. She swims 4.2 km up the river in 14 minutes. How much time will she take to swim 18.4 km down the river?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 46 min
Write the speeds as \(7k\) (still water) and \(k\) (river). Up the river she swims at \(7k-k=6k\) and down the river at \(7k+k=8k\).
Upstream speed \(\displaystyle =\frac{4.2}{14}=0.3\) km per minute \(=18\) km/hr
\(6k=18\), so \(k=3\)
Downstream speed \(=8k=24\) km/hr \(=0.4\) km per minute
Time \(\displaystyle =\frac{18.4}{0.4}=46\) minutes