Help please! - Bracket Maths

[lakshmi]

In a question of this format, (a [ b ] c), should the number in middle should always be made using numbers on either side or can 'b' be made using just either a or c? And can a, b, c can be a sequence as well?

In this question,

(36 [18] 6) (24 [12] 4) (42 [ ? ] 7)

my DD worked out the answer as 21 (half of the first number or 3 times the third number is the rule according to her.) When I suggested that she should use both the numbers, she came up with the rule saying it is a sequence with 1/2 and 1/3. Is that ok?

My rule of "first number - 3 x the third number" was not very appealing to her:-((

[aargh]

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.
Your

"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?

[ian35mm]

21 is correct.

First number divided by three, plus the third number.

42/3=14

14+7=21

[lakshmi]

Thank you very much.