If 9 examiners can examine a certain number of answer books in 12 days by working 5 hours a day; for how many hours a day would 4 examiners have to work in order to examine twice the number of answer books in 27 days?

Question: If 9 examiners can examine a certain number of answer books in 12 days by working 5 hours a day; for how many hours a day would 4 examiners have to work in order to examine twice the number of answer books in 27 days?

Solution:
9 examiners need 12 days by working 5 hours
1 examiner needs 1 day by working 5 × 9 × 12 hours
4 examiners need 27 days by working (5 × 9 × 12)/(4 × 27) hours
= 5 hours

So, twice the work need twice time.
∴ So, the answer = (5 × 2) = 10 hours