Question 433814
Let the river current be x km/hr.
Then, speed of the boat is 2x km/hr.


Time taken to go 18km downstream = 18/(2x+x) = 6/x hr.


Time taken to go 18km upstream = 18/(2x-x) = 18/x hr.


Total time taken = 6/x + 18/x hr. 

6/x + 18/x = 8


24/x = 8


x = 3


Therefore, time taken for return trip (18km upstream) = 18/(2x-x) = 18/3 = 6 hr.