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 43 minutes of call is $19.84, and the remaining credit after 65 minutes of call is $17.20. What is the remaining credit after 79 minutes of call.

Answer: the remaining credit after 79 minutes of call is $15.52

Step-by-step explanation:

let x represent the number of minutes of call.

Let y represent the remaining credit after x minutes of call.

If we plot y on the vertical axis and x on the horizontal axis, a straight line would be formed. The slope of the straight line would be

Slope, m = (17.2 - 19.84) / (65 - 43)

m = - 2.64/22 = - 0.12

The equation of the straight line can be represented in the slope-intercept form, y = mx + c

Where

c = intercept

m = slope

To determine the intercept, we would substitute x = 65, y = 17.2 and m = - 0.12 into y = mx + c. It becomes

17.2 = - 0.12 * 65 + c

17.2 = - 7.8 + c

c = 17.2 + 7.8

c = 25

The linear function becomes

y = - 0.12x + 25

Therefore, the remaining credit after 79 minutes of call is

y = - 0.12 * 79 + 25

y = - 9.4 + 25

y = 15.52