Originally Posted by marslanqaisar

Question: A and B do a job together in 2 hours. A can do the same job alone in 5 hours. how long will it take by B to do the same job alone?

Solution:
For example, if A and B do a job together in 2 hours, then in one hour, 1/2 will get done.
In general, if a job takes x hours, then in one hour, 1/x will get done.
So, Let x be how long will it take B to do the same job alone.
Then
1/x is that fraction of the job that gets done in one hour.
Therefore,
A and B do a job together in 2 hour = 1/2 of the job
and
A do the same job in 5 hours = 1/5 of the job
.
We have,

1/x = 1/2 + 1/5

1/x = 5+2/10
1/x = 7/10
Therefore, on taking reciprocals,
x = 10/7 hours

I tried to solve this (may be wrong) and correct me if I am wrong.