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     we put the 10 pencil boxes on the both sides of a weighing balance.  The side which is heavier will have pencils of 10gms.  The side which has pencils of 9 gm, will be lighter and will go up.

     wehn we weigh the two boxes against each other, 90 gm and 10gm are weighed against one another. so the lighter one will be the 90 gm box.

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