Question

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 24 minutes of calls is $21.64, and the remaining credit after 35 minutes of calls is $20.10. What is the remaining credit after 45 minutes of calls?

Answer 1

The remaining credit after 45 minutes of calls is $18.70

Explanation:

Here, we have a linear model to work with

Let us have the coordinates as follows;

(time calling time, credit remaining)

So we have two points as follows;

(24,21.64) and (35,20.10)

The slope will be;

(20.1-21.64)/(35-24) = -1.54/11 = -0.14

So the equation looks like;

y = -0.14x + c

To get the y-intercept, we simply substitute a point

Let us use (35,20.10)

20.10 = -0.14(35) + c

c = 20.10 + 4.9

c = 25

So the full equation is;

y = -0.14x + 25

So for 45 minutes, simply substitute 45 for x

We have this as:

y = -0.14(45) + 25

y = -6.3 + 25 = $18.70