Suppose, m be the number of minutes felipw used.
For his phone service, Felipe pays a monthly fee of $20, and he pays an additional $0.04 per minute of use.
Therefore, the total amount paid by him in a month is

The least he has been charged in a month is $78.04.
Therefore,

So, the possible number of minutes he has used in a month is 1451 minutes or more than 1451 minutes.