I just did this question on the certification and I have  feeling it's like this:

Premise: Class A income + Class B income = Total income

Class A cost allocation = Class A Income / Total Income

Class B cost allocation = Class B Income / Total Income

Expenses: Class E1, E2, E3, and so on

Example journal entry to allocate costs

                                                debit            credit

Class E1 Account                      X1                           (Class = Class A)

Class E1 Account                      X2                           (Class = Class B)

                Class E1 Account                          X3       (Class = OVERHEAD)

where:

X1 = Class E1 Total Expense * Class A cost allocation

X2 = Class E1 Total Expense * Class B cost allocation

X3 = X1 + X 2  (the original Expense transaction amount)

Will update the thread if it's wrong