There are nine silver coins marked 1, 2, 3, 4, 5, 6, 7, 8, and 9.
They are supposed to weigh the number of grams that is written on them.
One of the coins is fake and is lighter than it should be.
Find the fake coin using the balance scale twice.
(In reply to Two preplanned weighings
by Brian Smith)
Excellent, elegant approach!
Unfortunately, this specific preplanned weighing fails, because 1+3+7 < 2+5+6.
However, it would work if the 2nd weighing was 2+3+7 vs 1+5+6