Find time taken for boat in still water When it 8hrs in down streem and 12 hrs in up stream?

Other answer:

Hi it can't be still water if it is a stream there must be a current flowing as upstream will take longer against a flow down stream.
Dan B:
10 hrs
It will be the average of the two times.
let r be your rowing speed, c be the current speed, let d be one-way distance. assume you start upstream.
rowing down stream the distance is d, the time is 8, the net speed is r+c. 8 = d/(r+c)
coming back, distance is d, time is 12, net speed is r-c. 12= d/(r-c)

in still water, distance is d, net speed is r, time is d/r.

so if 8 = d/(r+c), and 12 = d/(r-c), solve for d/r.

Dan is quite correct in his answer.