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A gardener measures the heights of 30 sunflower seedlings…

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

A gardener measures the heights of $30$ sunflower seedlings. The table shows the result.

Height (cm)	Seedlings
10-15	2
15-20	5
20-25	9
25-30	8
30-35	6

Find the class mark of the class $20$ to $25$ , and the mean height by the direct method.

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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