From the given statement:

To determine the fraction amount of the classes that like oatmeal, we add the fraction that likes oatmeal from each class.

To add the fraction, the first step is to find the lowest common multiple (L.C.M) of the denominators.
L.C.M of 3 and 4 is 12
Therefore:

We can then say that the fraction amount of the classes that like oatmeal is 11/12.