The monthly cost, in dollars, of a cell phone bill is

P = f(t, m, d) = 0.25t + 0.2m + 0.01d

where t is the number of minutes talked, m is the number of messages sent and d is the number of kilobytes of data used that month.
(a) Find f(250, 200, 100) and interpret it.
(b) Find a formula for monthly cost if your data is disabled.
(c) You are considering switching to an unlimited talk, text and data plan at $50 per month. If your average usage is t = 120, m = 100 and d = 250, does it make sense to switch?