Recall direct method of finding mean.

Can we find the upper and lower limit of a class interval?

Think about the direct mean formula.

Let us list the first 10 even numbers.

Subtract 0.5 from lower llimit and add 0.5 toÂ the upper limit.

We can find the class mark by adding upper limit and lower limit divided by two.

Average is a central value.

First find its class mark, and put all the values in the formula of step deviation.

Think of formula of mean.