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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.

boxes and scales

What will you weigh?