You have ten boxes; each contains nine balls.

logic/weighing/box

Problem

You have ten boxes; each contains nine balls.  The balls in one box
weigh 0.9 kg; the rest weigh 1.0 kg.  You have one weighing on an
accurate scale to find the box containing the light balls.  How do you
do it?

Show solution

Number the boxes 0-9.  Take 0 balls from box 0, 1 ball from box 1, 2
balls from box 2, etc.  Now weigh all those balls and follow this
table:

If odd box is        Weight is
0                45 kg
1                44.9 kg
2                44.8 kg
3                44.7 kg
4                44.6 kg
5                44.5 kg
6                44.4 kg
7                44.3 kg
8                44.2 kg
9                44.1 kg