A can do a job in 16 days, B can do same job in 12 days. With the help of C they did the job in 4 days. C alone can do the same job in how many days ?

Correct Answer : A.

In this question we having, A's work, B's work and A+B+C work. We need to calculate C's work.
We can do it by,
(A+B+C)'s work - (A's work + B's work).
Let's solve it now:
C's 1 day work =    
                            $$\begin{gathered} \frac{1}{4}- \left(\frac{1}{16}+\frac{1}{12}\right) \\ = \left(\frac{1}{4}-\frac{7}{48}\right) \\ =\frac{5}{48} \end{gathered}$$
So C can alone finish the job in 48/5 days,
Which is =  $9\frac{3}{5}days$