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)