Discussion :: Time and Work

A can lay railway track between two given stations in 16 days and B can do the same job in 12 days. With help of C, they did the job in 4 days only. Then, C alone can do the job in:
Answer : Option C
Explanation :
(A + B + C)'s 1 day's work =\(\frac {1} {4} \)
A's 1 day's work =\(\frac {1} {16} \)
B's 1 day's work =\(\frac {1} {12} \)
C's 1 day's work =\(\frac {1} {4} \)[\(\frac {1} {16} \)+\(\frac {1} {12} \)]=[\(\frac {1} {4} \)\(\frac {7} {48} \)]=\(\frac {5} {48} \)
So, C alone can do the work in\(\frac {48} {5} \)=9\(\frac {3} {5} \)days
How you get 48/5
how we put 48/5?