How many hours to complete the job?

Thirty men take 20 days to complete a job working 9 hours a day.How many hour a day should 40 men work to complete the job?

Answer: 

6.75hours

Before.....

one day work=1/20

one hour work=1/(20x9)

one men work=1/(20x9x30)

now....

one day work=1/20

one hour work=1/(20xY)

one men work=1/(20xYx40)

ie, 1/(20x9x30) = 1/(20xYx40)

after calculation Y=27/4

Y=6.75hours.