let the number of marbles be as follows:
after the described action
box1 = x/3 (you took 2/3 away, so 1/3 remains)
box2 = 129-x-y + 15 = 144-x-y
box3 = 3y
then (x/3) : 144-x-y : 3y = 2:3:9
so then (x/3)/(144-x-y) = 2/3
3x + 2y = 288 (equ#1)
also (x/3)/3y = 2/9
x = 2y (equ#2) sub that into equ#1 to get
y = 36
then x = 72
box3 36 --- total 129
box1 = 24
box2 = 36
box3 = 108
24:36:108 = 2:3:9
After: 24:36:108 (total 168)
Before: 72:21:36 (total 129)
Method used: trial and error
Start with "after" combinations with the right ratios, determine the required operations and "before" compositions, and see which one adds up to 129 "after".
It only took three attempts.
simply just wat is says above 168
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