Hi;
1 - 10, 10 - 20 ect if these are class limits are the boundaries 20/2 = 10(lower boundary for second class)
40/2 =20(upper boundary for second class).
In short, the boundaries are what you make it. If one class contains [1,9] and the other is [10,19] then the boundaries relate to the smallest and largest values of each class.