A and B took a job to be completed in 20 days. They started working together and after 12 days, C joined them and the whole job finished in 15 days. How much time would C require to complete the job if only C was hired?

**(A)** 15

**(B)** 12

**(C)** 10

**(D)** 8

**Answer:** **(B)** **Explanation:** Let the total job be 20 units. It is given that A and B took the job to be completed in 20 days.

=> Combined efficiency of A and B = 20/20 = 1 unit / day

Now, job done in 12 days = 12 units

=> Job Left = 8 units

Now, this remaining 8 units of job has been done by all A, B and C together.

Let the efficiency of C be ‘x’.

=> Combined efficiency of A, B and C = 1+x units/ day

Now, with this efficiency, the job got completed in 3 more days.

=> Job done in 3 days = 3 x (1+x) = 8 units

=> x = 5/3

Therefore, efficiency of C = x = 5/3 units / day

Hence, time required by C alone to do the job = 20/(5/3) = 12 days

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