Combine work

IMO 18 hours
Let me know if i am right , i will let you know how i have done

Actually I only have an answers and dont know the right answer. )

IMO 18 hours.
you can always solve it by algebra but I love to do reasoning.
so together they finish in 5 hours.
if john had worked alone he would have finished in 6 hours.
so Jim did in 3 hours what john does in an hour
in one hour john does 1/6 of work.
so to finish the work john will take 3*6 hours(coz 6*1/6=1)

Yeah, 18 hours is what I got too.

Alright, so John takes 6 hours to complete a task and 2 hours after he started, his friend, Jim comes along and begins to help him. Working together, they finish the remaining task in 3 hours.

So, John has already finished 2/6 or 1/3 of the task, so it'll take him 4 hours to finish the remaining 2/3 of the task.

1/4+1/Jim=1/3

Jim= The number of hours it'll take Jim alone to complete 2/3 of the task.

1/3-1/4=1/Jim

1/12=1/Jim

Jim=12 hours to finish 2/3 of the task

It'll take Jim 6*3 or 18 hours to finish the whole task working alone.