<PRE><B>Question 1086496</B>
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 
25 minutes of calls is $26.50, and the remaining credit after 53 minutes of calls is $22.58. What is the remaining credit after 61 minutes of calls ?

Solution:
Given linear function, we have two given points of 
f(n), namely
(25,26.50), (53,22.58)
So apply the two point formula:
slope m=(y1-y1)/(x2-x1)
=(22.58-26.5)/(53-25)
=-3.92/28
=-0.14
 
the equation required is
(y-y1)=m(x-x1)
y-26.5=-.14(x-25)
simplify to get
y=-0.14x+30
 
The amount left after 61 calls
A=f(61)=-0.14*61+30=21.46</PRE>