A mortor-boat whose speed is 30km/hr in water goes 30km downstream and comes back in a total time of 4hrs 30 minutes find the speed of the stream.
Here is an outline:
Let the speed of the current be w km/hour
Going downstream the speed is 30+w
so the time is 30/(30+w)
Going upstream the speed is 30-w
so the time taken will be 30/(30-w)
If you add the time to go downstream to the time to go upstream the total must be 4.5 hours.
So make the equation and solve it for w.
Then you can plug w back in to check that it works.
Please let guest finish this for him/herself.
If you want to ask questions you can but you must show evidence of your own contribution as well.