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Solution: The left-side box is yellow.

Here’s why:

• Initially:
  • Green box = 2 × Yellow box.
• When the RD takes 5 copies out of the left-side box, the contents decrease by 5.
• Then, half of the remaining copies in the left-side box are moved to the red box.
• For this to be possible, the left-side box must have started with at least 10 copies (since half must be a whole number).
• Since the green box originally had twice as many plans as the yellow box, this implies:
  • Green box had an even number of copies.
  • Therefore, the yellow box must be the left-side box.
  • After removing 5 copies, there would still be an even number of plans remaining, making it possible to move half to the red box.

Solution: The left-side box is yellow.