Answer:
30 +0.45(m -500) . . . . for m > 500
Step-by-step explanation:
If talk time is "m" minutes, then the number of "additional minutes" is ...
m -500
The cost of those is $0.45 each, so (in dollars), the charge for additional minutes is ...
0.45(m -500)
The charge of $30 applies regardless of the number of minutes, so is added to the minutes charge. The total cost of a bill for talking more than 500 minutes is ...
30 +0.45(m -500)
_____
The cost for talking 500 minutes or less is simply 30.
