Norma finds her mean number of minutes per call to be 27, with a standard deviation of 6 minutes. The minutes per call are approximately normally distributed around 27 with no outliers.

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27 To find the probability of a specific value or range of values for a normally distributed variable, such as the number of minutes per call in this case, you can use z-scores and the standard normal distribution.

The standard normal distribution has a mean of 0 and a standard deviation of 1. However, in this case, Norma's mean is 27 and the standard deviation is 6.

To find the z-score for a specific value, you can use the formula:

z = (x - mean) / standard deviation

where x is the specific value, mean is the mean of the distribution, and standard deviation is the standard deviation of the distribution.

Let's say you want to find the probability of a call lasting less than 20 minutes. To find the z-score for 20 minutes, you would use the formula:

z = (20 - 27) / 6 = -1.167

Once you have the z-score, you can use a standard normal distribution table or calculator to find the corresponding probability. The z-score represents the number of standard deviations below (negative z-score) or above (positive z-score) the mean.

For example, if you look up the z-score of -1.167 in a standard normal distribution table, you would find that the corresponding cumulative probability is 0.121, or 12.1%.

Therefore, the probability of a call lasting less than 20 minutes is approximately 0.121, or 12.1%.

Similarly, you can use this method to calculate probabilities for other scenarios, such as finding the probability of a call lasting between two specific values. Just find the z-scores for the lower and upper values, and then subtract the cumulative probabilities to find the probability of it falling within that range.

Remember that this method assumes a normal distribution and no outliers, so it may not be reliable if those assumptions are not met.