This example involves using the Normal probability distribution as an approximation to the Binomial distribution.

You can look up the Standard Normal Probability table, for example at http://www.statsoft.com/textbook/sttable.html where you will also find some of the theory about the distribution, or you can use an online calculator which will give you readings automatically, for example the one at http://davidmlane.com/hyperstat/z_table.html .

In this example you are not given the number of tickets sold and asked to find the probability that too many passengers will turn up for the flight, but rather the inverse problem, and solving this requires some trial and improvement method.