Posted by Stephanie on .
A man selling fruit has only three weights. But with them he can weigh any whole number of kilograms (from 1 kg to 13 kg) inclusive on his balance.
What weights does he have?
Responses
math  Kalyan, Friday, September 5, 2008 at 4:38am
The man had 1kg, 3kg and 9 kg weights.
How do you set it up to get this answer? Any ideas?

Math 
drwls,
You can prove to yourself that 1,3 and 9 work. With them, you can form counterbalances of
1, 2 (31), 3, 4 (3+1), 5 (913),6 (93), 7 (9+13), 8 (91), 10 (9+1), 11 (9+31), 12 (9+3) and 13 (9+3+1)
A minus sign means putting the weight on the same side of the balance as the weighed object.
Proving that other combinations will not work is harder, but is not necessary.