Question

A study of long-distance phone calls made from General Electric Corporate Headquarters in Fairfield, Connecticut, revealed the length of the calls, in minutes, follows the normal probability distribution. The mean length of time per call was 4.2 minutes and the standard deviation was 0.40 minutes. What is the probability that calls last between 4.2 and 4.9 minutes

Answer 1

Answer:

The probability that a call last between 4.2 and 4.9 minutes is 0.4599

Step-by-step explanation:

Let X be the length in minutes of a random phone call. X is a normal distribution with mean λ=4.2 and standard deviation σ=0.4. We want to know P(4.2 < X < 4.9). In order to make computations, we will use W, the standarization of X, given by the following formula

$W = \frac{X-\mu}{\sigma} = \frac{X-4.2}{0.4}$

We will use $\phi$ , the cummulative distribution function of W. The values of $\phi$ are well known and the can be found in the attached file

$P(4.2 < X < 4.9) = P(\frac{4.2-4.2}{0.4} < \frac{X-4.2}{0.4} < \frac{4.9-4.2}{0.4}) = P(0 < W < 1.75) = \\ \phi(1.75) - \phi(0) = 0.9599-0.5 = 0.4599$

We conclude that the probability that a call last between 4.2 and 4.9 minutes is 0.4599

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