Question 1111736
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You have a linear function with the values of

    {{{y[1]}}} = 16.64  at  x = 28,   and

    {{{y[2]}}} = 12.20   at  x = 65.


Hence, the slope of the straight line is

    m = {{{(y[2]-y[1])/(x[2]-x[1])}}} = {{{(12.20-16.64)/(65-28)}}} = -0.12.


    // Surely, you do understand that  |m| = |-0.12| = 0.12 dollars is the cost rate (= price) for one minute of the phone conversation.



Now the value of  "y" at x= 89 minutes is

    y = {{{y[2]}}} + {{{m*(89-x[2])}}} = 12.20 -0.12*(89-65) = 9.32 dollars.


<U>Answer</U>.  The remaining credit after 89 minutes of calls is 9.32 dollars.
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