The credit remaining on a phone card (in dollars) is a linear function of the total calling time made with the card (in minutes). The remaining credit after 42 minutes of calls is $23.70, and the remaining credit after 53 minutes of calls is $22.05 . What is the remaining credit after 61 minutes of calls?

The remaining credit after 61 minutes of calls is $20.85.

Step-by-step explanation:

The credit remaining after t minutes can be modeled by a linear function is the following format:

In which C(0) is the initial amount of credit and a is the cost of a minute.

The remaining credit after 42 minutes of calls is $23.70

This means that

So

The remaining credit after 53 minutes of calls is $22.05.

This means that

So

From above:

So

So

What is the remaining credit after 61 minutes of calls?

This is C(61).

The remaining credit after 61 minutes of calls is $20.85.