Along distance telephone plan has a monthly fee of $15.00 and a rate of $0.05 per minute. how many minutes can you chat long distance in a month for a total cost, including the $15.00, of $45.00?

You know that no matter what, you will pay 15 dollars during the month. 
45-15=30, so you have 30 dollars left to spend on talking. Now you divide 30 by 0.05 to find the amount of minutes possible. 30/0.05=600, so you can spend 600 minutes talking.