There are 2187 balls, out of them 1 is heavy. Find out the heavy ball with minimum no. of attempts to weighed the balls?

Question

There are three keys to the three doors with different locks on them.

How many attempts would be needed to determine the exact keys for all the doors under all circumstances?

Answer 1

after 7 weighings

1.Devide the balls into 3 piles(729 balls) and weigh 2 piles, we can find the heavier pile.
2. Then devide the 729 balls into 3 piles(243 balls) and weigh 2 piles, we can find the heavier pile.
3. Then devide the 243 balls into 3 piles(81 balls) and weigh 2 piles, we can find the heavier pile.
4. Then devide the 81 balls into 3 piles(27 balls) and weigh 2 piles, we can find the heavier pile.
5. Then devide the 27 balls into 3 piles(9 balls) and weigh 2 piles, we can find the heavier pile.
6. Then devide the 9 balls into 3 piles(3 balls) and weigh 2 piles, we can find the heavier pile.
7. Weigh 2 balls of the 3 balls. We can find one hevier ball.

Excuse my poor English.