12 people can complete a job in 18 days, after 6 days 4 people are added. How long will it take them to finish the job?

Solution:

The time to finish the job is x

work done for 1 people of 1 day = 1/(12*18) = 1/216

Work left after 6 days = 2/3

Work done each day of 16 people = 16/216 = 2/27

So

x(2/27) = 2/3

x = 9

So it will take 9 days to finish the job