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'Money Measure' printed from https://nrich.maths.org/
You have ten identical open-topped boxes with 10 visually identical coins
in each.
In nine of the boxes each of the 10 coins has a mass
of 10g.
In one box the 10 coins have masses of only 9g each.
How can you find which box is the odd one out?
You have a normal mass measurer with a single pan and a scale. With just
one weighing you can identify the box with the lighter coins.
What will you weigh?