Question 431388
Let s = the rate in still water
Let c = the rate of the current = 6 km/hr
Let t = time
So the upstream rate = s - c
And the downstream rate = s + c
Since time = distance/rate, we can write
t = 7/(s-c)
t = 20/(s+c)
Equating the RHS, and subsituting the value for c gives:
7/(s-6) = 20/(s+6)
Cross multiply:
7s+42 = 20s-120 
Solving for s gives:
13s = 162 -> s = 12.462 km/hr