Outside of your particular text, the notion of "class midpoint" has no particular definition. The question cannot be answered without knowing what it is that you want to compute.
There are several possibilities.
1. The mean is the sum of class values divided by the number of them.
2. The median is the value having an equal number of class members above it as below it. (It might be the middle value, or the average of the two middle values.)
3. The midpoint of the range is the average of the highest and lowest class values.
4. Some other definition.
There are several possibilities.
1. The mean is the sum of class values divided by the number of them.
2. The median is the value having an equal number of class members above it as below it. (It might be the middle value, or the average of the two middle values.)
3. The midpoint of the range is the average of the highest and lowest class values.
4. Some other definition.
