Question

Ramu and Balu working together can paint a house in 20 days. Ramu alone can do it in 30 days how many days will balu take to complete the same work alone?

Answer 1

both Sonu and Neenu together can do a piece of work in 20 days.
therefore work done by both of them together in 1 day = 1/20
Sonu can work alone in 30 days.
the work done by Sonu in 1 day = 1/30
the work done by Neenu in 1 day = 
thus Neenu will take 60 days to complete the work alone.
hope this helps you.
cheers!!

Answer 2

No. of days taken by Ramu alone to do the work=30

Work done by Ramu in a day=1/30

No. of days taken by Ramu and Balu together=20

Work done by  Ramu and Balu in a day=1/20

Work done by Balu alone in a day=

Work done by them together in a day - Work done by Ramu alone in a day

1/20-1/30----------->3-2/60---------->1/60

Work done by Balu alone in a day=1/60

No. of days taken by Balu alone=60 days.