Question

How do I calculate the lower class boundary and the upper class boundary for the class 40 - 59?

Answer 1

It depends whether your variable is discrete or continuous .

A discrete variable can only have certain values, whole numbers in your case. Think of number of students in a certain course.
If this is the case the class goes from 40 to 59, and the class midpoint will be 49.5

A continuous variable can have any value.
Think of age, where "`59`" may mean "`60` minus one minute"
In this case the upperboundary will be `59.999...` and midpoint will be `50`.

A third case is when the values have been rounded , like exam scores. There `40` would mean `39.5` and up, so that would be your lower boundary. `59` would mean up to (not including) `59.5`

So it really does depend on the kind of data that you have.