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Math Help - Stat word problem need help URGENT

  1. #1
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    Exclamation Stat word problem need help URGENT

    Titan technology is a tech company that is a new cell phone service provider. Their rate for any phone call is determined as follows: the first minute costs $.15 and every minute thereafter costs $.01. Statisticians who work for Titan Technology collected information for a month regarding the costs of phone calls. The costs were normally distributed with a mean of $.25 and a standard deviation of $.03.

    A. How much would a 20 minute phone call cost?

    I got 34 cents but am not sure

    B. Write a formula to represent the cost of a phone call. Use "c" to represent cost and "m" to represent the number of minutes.

    C. A rival tech company, Idealistic Innovations, charges a flat rate of $.29 per call. Determine when a phone call would cost the same using Idealistic and using Titan.

    D. Using what you learned in part C determine the percent of calls made my Titan Technology customers that would be cheaper if they had used Idealistic Innovations instead.
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  2. #2
    Pim
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    A. That's correct

    B. It's a linear formula in the form of c = am + b
    a = 0.01 and b = 0.15
    Therefore

    c = 0.01m + 0.15

    C. 0.29 = 0.01m +0.15
    0.14 = 0.01m
    14 = m

    The phonecalls cost the same amount at 14 minutes.

    D.
    <br /> <br />
P(X > .29) = \frac{1}{\sqrt{2\pi}}\int_\infty^{.29} e^{-u^{2}/2}du<br /> <br />

    Which is the area under normal curve from 0.25 to  \infty - the area from 0.25 to 0.29

    0.29 in standard units: (0.29 - 0.25) / 0.3 =  1 \frac{1}{3}

    Which yields from the standard distrbution table: 0.5 - 0.4082 = 0.0918, which gives 9.18%
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