Ankit went to Manali, for river rafting. He rows at the rate of 12 km/h in still water. To cover the same distance, it takes him twice as long to row up as to row down the river. Find the rate of flow of the river.

4 km/h

If we let the speed of the boat = x and the speed of the stream = y then,

Speed of downstream = (x + y) km/h

Speed of upstream = (x – y) km/h

We have given,

Speed of boat in still water = 12 km/h

Time taken for upstream = 2 × Time taken for downstream

So,

Let the rate of the stream be 's' km/h and the distance traveled be 'd' km.

Rate of downstream = (12 + s) km/h

Rate of upstream = (12 – s) km/h

According to the question,

= \(\frac{2d}{12 + s}\)  =  \(\frac{d}{12 - s}\)

= 24 – 2s = 12 + s

= 3s = 12

= s = 4 km/h