For a basic monthly fee of F yen ($F), Naoko's first cell phone plan allowed him to use a maximum of 420 minutes on calls during the month. Then, for each of x additional minutes he used on calls, he was charged $M, making his total charge for the month $T, where T = F + xM. What is the value of F?
(1) Naoko used 450 minutes on calls the first month an the total charge for the month was $13,755.
(2) Naoko used 400 minutes on calls the second month and the total charge for the month was $13,125.