How do you calculate class boundary?

Class boundaries are defined as the average of the upper limit of one class and the lower limit of the next class.

For example:

No. of runs (class)

0-4

5-10

10-15

15-20

and No. of batters (frequency)

3

9

10

4

So the upper class boundary for the first class is the average of 4 (upper limit of class 0-4) and 5 (lower limit of next class 5-10), i.e. 4.5.

Similarly, the next boundaries are 10.5, 15.5 and 20.5.

In this way, there's no gap between 2 bars of a histogram, i.e., in this example the bars range from:

0-4.5

4.5-10.5

10.5-15.5

15.5-20.5

(Hope this helps! Took me a while to understand this stuff too :D)