A can do a job in 12 days and B can do the same job in 10 days. With the help of C they can do the same job in 4 days. In how many days C alone can do this job?

Question: A can do a job in 12 days and B can do the same job in 10 days. With the help of C they can do the same job in 4 days. In how many days C alone can do this job?

Solution:
A's one day work = 1/12

B's one day work = 1/10

(A+B+C)'s one day work = 1/4

Therefore, C's one day work = (A+B+C)'s one day work - (A+B)'s one day work
So, C's one day work = 1/4 - (1/12 + 1/10) = 1/4 - {(5 + 6)/60} = 1/4 - 11/60 = (15 - 11)/60 = 4/60 = 1/15

So, C will complete the work in 15 days.