answer:It’s very straightforward. Once you know how to do it, you can accomplish many things. First, the most important thing is the standard cumulative Normal distribution. With a normal distribution of mean 0 and stddev of 1, the value f(X) is the area under the curve from “negative infinity” to X. This in turn is the percentage of data points with a z-score below X, or in other words the percentage of points less than X. Now, you must calculate the z-scores for the two points in question. z-score = (X – mean)/stddev (40 – 40)/5.5 = 0 (51 – 40)/5.5 = 2 Use a chart such as this to find the value of the cumulative distribution at those points. For 2, the value is .9772, and for 0 the value is 0.5. p(40 < time < 51) = p(0 < z < 2) = p(z < 2) – p(z < 0) = 0.9772 – 0.5 = 0.4772 So, about 47.72% of people, or about 143 people, spend between 40 and 51 minutes.