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 41 minutes of calls is $23.85, and the remaining credit after 74 minutes of calls is $18.90. What is the remaining credit after 102 minutes of calls?

Answer 1

Answer:

Knowing that thet credit decrease linearly, you can use the equation of a straight line to find the answer

Step-by-step explanation:

We will say that the time is 't', and the remaining money is 'm'.

The equation of an straight line is like this:

(y2-y1)=m*(x2-x1)

Where x2 and x1 are two values of the horizontal axis, y2 and y1 are the image of x2 and x1 in the vertical axis, and m is the slope.

For our problem we will say that 't' is in the horizontal axis and 'm' is in the vertical one, as m decreases with t. So:

(m2-m1)=m*(t2-t1)

Now we can substitute with the two pair of values that we already have to find the slope of the straight line:

(18.90-23.85)=m(74-41)

Once we have got the slope, we can do the same but with another point, as the slope is always the same:

(m3-m1)=m*(t3-t1)

Our third point is the one which t=102min so our incognit will be its image, m3:

(m3-23.85)=m*(102-41)

We have it all but m3, so we can solve it