Answer:
(a) The percent of her laps that are completed in less than 130 seconds is 55%.
(b) The fastest 3% of her laps are under 125.42 seconds.
(c) The middle 80% of her laps are from <u>126.80</u> seconds to <u>132.63</u> seconds.
Step-by-step explanation:
The random variable <em>X</em> is defined as the number of seconds for a randomly selected lap.
The random variable <em>X </em>is normally distributed with mean, <em>μ</em> = 129.71 seconds and standard deviation, <em>σ</em> = 2.28 seconds.
Thus,
.
(a)
Compute the probability that a lap is completes in less than 130 seconds as follows:
![P(X](https://tex.z-dn.net/?f=P%28X%3C130%29%3DP%28%5Cfrac%7BX-%5Cmu%7D%7B%5Csigma%7D%3C%5Cfrac%7B130-129.71%7D%7B2.28%7D%29)
![=P(Z](https://tex.z-dn.net/?f=%3DP%28Z%3C0.13%29%5C%5C%3D0.55172%5C%5C%5Capprox0.55)
The percentage is, 0.55 × 100 = 55%.
Thus, the percent of her laps that are completed in less than 130 seconds is 55%.
(b)
Let <em>x</em> represents the 3rd percentile.
That is, P (X < x) = 0.03.
⇒ P (Z < z) = 0.03
The value of <em>z</em> for the above probability is:
<em>z</em> = -1.88
Compute the value of <em>x</em> as follows:
![z=\frac{x-\mu}{\sigma}\\-1.88=\frac{x-129.71}{2.28}\\x=129.71-(1.88\times 2.28)\\x=125.4236\\x\approx 125.42](https://tex.z-dn.net/?f=z%3D%5Cfrac%7Bx-%5Cmu%7D%7B%5Csigma%7D%5C%5C-1.88%3D%5Cfrac%7Bx-129.71%7D%7B2.28%7D%5C%5Cx%3D129.71-%281.88%5Ctimes%202.28%29%5C%5Cx%3D125.4236%5C%5Cx%5Capprox%20125.42)
Thus, the fastest 3% of her laps are under 125.42 seconds.
(c)
Let <em>x</em>₁ and <em>x</em>₂ be the values between which the middle 80% of the distribution lie.
That is,
![P(x_{1}](https://tex.z-dn.net/?f=P%28x_%7B1%7D%3CX%3Cx_%7B2%7D%29%3D0.80%5C%5CP%28-z%3CZ%3Cz%29%3D0.80%5C%5CP%28Z%3Cz%29-P%28Z%3C-z%29%3D0.80%5C%5CP%28Z%3Cz%29-%5B1-P%28Z%3Cz%29%5D%3D0.80%5C%5C2P%28Z%3Cz%29%3D1.80%5C%5CP%28Z%3Cz%29%3D0.90)
The value of <em>z</em> for the above probability is:
<em>z</em> = 1.28
Compute the values of <em>x</em>₁ and <em>x</em>₂ as follows:
![z=\frac{x_{2}-\mu}{\sigma}\\1.28=\frac{x_{2}-129.71}{2.28}\\x_{2}=129.71+(1.28\times 2.28)\\x=132.6284\\x\approx 132.63](https://tex.z-dn.net/?f=z%3D%5Cfrac%7Bx_%7B2%7D-%5Cmu%7D%7B%5Csigma%7D%5C%5C1.28%3D%5Cfrac%7Bx_%7B2%7D-129.71%7D%7B2.28%7D%5C%5Cx_%7B2%7D%3D129.71%2B%281.28%5Ctimes%202.28%29%5C%5Cx%3D132.6284%5C%5Cx%5Capprox%20132.63)
Thus, the middle 80% of her laps are from <u>126.80</u> seconds to <u>132.63</u> seconds.