You are supposed to have to use both the outside numbers to make the middle one.
The method you use has to be identical for all 3 sets.
You cannot have an either or answer - that leads you to the "trick" answer options on the multiple choice which are there to catch the unwary.
For the example you give, your DD's solution seems to work but does not combine the 2 outside numbers, there must therefore be another method going on.
"first number - 3 x the third number" was not very appealing to her
is correct because it uses both side numbers (which is essential).
It seems a badly written question to me because of the particular choice of numbers.
Can you post the four options from the answer sheet?