So.. the boat in still water runs at 9km/h, so that's its rate
going from N upstrea, going against the current's rate, you guessed it, if the stream has a rate "r", then the boat is going up at 9 - r
now, he started out at point N, and stop rowing, and backslid to point N
the way back he was just going at the current's speed or "r"
notice, he started from point N, went up 6km, then came back to point N, meaning, the distance one way is 6km, and back is the same 6km
now, since he started from point N, till he came back, the length of the whole trip forth and back is 2hrs and 40mins, or 160minutes
well, you know that d = rt
thus
I'd think you know how to get "r" by now
going from N upstrea, going against the current's rate, you guessed it, if the stream has a rate "r", then the boat is going up at 9 - r
now, he started out at point N, and stop rowing, and backslid to point N
the way back he was just going at the current's speed or "r"
notice, he started from point N, went up 6km, then came back to point N, meaning, the distance one way is 6km, and back is the same 6km
now, since he started from point N, till he came back, the length of the whole trip forth and back is 2hrs and 40mins, or 160minutes
well, you know that d = rt
thus
I'd think you know how to get "r" by now