There are 27 identical balls. In that one of the balls is overweight. There are only three chances to weigh in the weighing machine. How to find the overweight ball?
First weigh 9 against 9 balls. If the ball contains among these 18 balls, then take the nine balls which weighed over, and weigh the ball 3 against 3.
Among these six balls, if any one of the side weighed more, take the three balls and weigh 1 against 1. If the balls are not equal and any one of the ball weighs more, that is the overweight ball, else the one among the three which is not weighed is the ball.
If the 3 against 3 balls were equal, then weigh the other 3 balls, then the overweight ball will be found. If the 9 against 9 balls are equal, then take the other 9 balls, weigh 3 against 3 and 1 against 1 to find out the overweighed ball.