Its about linear function

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
38
minutes of calls is
$25.06
, and the remaining credit after
65
minutes of calls is
$21.55
. What is the remaining credit after
74
minutes of calls?

 

Answer 1

Since it's a linear function, the $/minute remains constant.

 

So the constant function or slope, can be found by using this equation.

 

(beginning $ - ending $)/time elapsed

 

= ($25.06 - $21.55)/(65 min - 38 min)

=  $3.51/27 min

=  $0.13/min

 

So, for the last part you can see the person spoke for another 9 minutes.

 9 min x $0.13/min = $1.17

 

If you deduct $1.17 from the balance of $21.55, you're left with $20.38