This is a question about math not finance but i will use the following formula to illustrate:
A) Total return = Dividend yield + Price change
B) Price change = (1 + change in 'price over earnings multiple') * (1 + change in 'earnings')
C) Price change = Change in 'price over earnings multiple' + Change in 'earnings'
How can you explain that (C) is incorrect and (B) should be used instead given (B) will ultimately get you the correct answer?
Cheers!
The question
is neither mathematics nor finance. It's pedagogy. The answer depends on what the person to whom you are explaining already knows.
I would guess that in the example you give the best strategy might be to work through a particular numerical example, showing the price change calculated both ways. WIth the numbers the explainee might see why (B) is correct.