Gee wrote:

I need help from Maths experts.

I don't know where to start.

Three boxes (A, B and C) contain red balls or yellow balls or both.

Each box contains the same number of balls.

Box A contains all twelve of the red balls and one-ninth of the yellow balls.

How many yellow balls are there altogether?

How many balls are there in each box?

Keep upping the number of yellow balls until that number, times 8 - to make the 'added' yellow balls one ninth of the total yellow balls - (or, that number, times 4) gives you the number of balls in total you now have in the first box? Then times the total in the first box by 3?

e.g. 12 red, add 3 yellow, 15 total in that box. 3 x 8, divided by 2, equals 24/2, so 12. Do not that, keep trying...