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Harry, Christine and Betty are packing $36$ boxes of chocolates.

Harry and Christine would take $2$ hours to pack the boxes;

Harry and Betty would take $3$ hours;

Christine and Betty would take $4$ hours.

How many boxes does Christine pack in one hour?

If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.

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This problem is taken from the UKMT Mathematical Challenges.