Question 9

# Swati can row a boat on still water at a speed of 5 km/hr. However, on a given river, it takes her 1 hour more to row the boat 12 km upstream than downstream. One day, Swati rows the boat on this river from X to Y, which is N km upstream from X. Then she rows back to X immediately. If she takes at least 2 hours to complete this round trip, what is the minimum possible value of N?

Solution

Let the speed of the stream be x

$$\frac{12}{5-x}=\frac{12}{5+x}+1$$

The value of x satisfying the above equation is 1

Now,

$$\frac{N}{5+1}+\frac{N}{5-1}\ge2$$

$$\frac{2N+3N}{12}\ge2$$

=> $$N\ge4.8$$