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
