Concept: The question tests on the application of basic arithmetic in a Word problem format.

Solution:

St (1)

 Given – There is an additional 30 minutes of use  (450-420=30) and 

                T = 13,755 

               Thus 13755 = F + 30 M

                We do not know F and hence we cannot compute the value of M. (Insufficient)

 

St (2) - Given T= 13,125 and since used minutes is less than 420,the value of x=0 (additional minutes)

Hence 13125 = F + 0 

  •  F = 13125

However the value of M cannot be computed as x = 0.  (Insufficient)

 

On combining both, we have 

13755 = 13125 +30 M

M can be computed (Sufficient)

Hence option (C)