The table shows the waiting times of 50 customers at a bank…

Contributed by StemPy Lab

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

The table shows the waiting times of $50$ customers at a bank. One frequency is missing.

Time (minutes)	Customers
0-5	8
5-10	14
10-15	12
15-20	9
20-25	?

Find the missing frequency, and then find the mean waiting time.

Need a reminder? The key ideaShow

For grouped data, each class is replaced by its class mark $x = \dfrac{\text{lower} + \text{upper}}{2}$ . The mean is

\displaystyle \bar{x} = \dfrac{\sum fx}{\sum f}.

For inclusive classes such as $1$ to $10$ , the class mark is still half the sum of the two ends.

The worked solution

Have you tried it?

Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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