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27 June, 02:16

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 43 minutes of call is $19.84, and the remaining credit after 65 minutes of call is $17.20. What is the remaining credit after 79 minutes of call.

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  1. 27 June, 02:43
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    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
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