8 balls puzzle

You have 8 balls. One of them is defective and weighs less than others. You have a balance to measure balls against each other. In 2 weighing, how do you find the defective one?

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ravi Expert Asked on 13th July 2015 in Interview Puzzles.
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    For convenience sake, let’s name the balls 1-8. First we weigh {1,2,3} on the left and {4,5,6} on the right. There are three scenarios which can arise from this.

    If the left side is lighter, then we know that one of 1, 2 or 3 is the lighter ball. Weigh {1} on the left and {2} on the right. By doing this, we will know if 1 or 2 is lighter. If they balance out, then 3 is the lighter one.

    If the right side is lighter, then we know that either 4, 5 or 6 is the lighter ball. Weigh {4} on the left and {5} on the right. By doing this we will know if 4 or 5 is lighter. If they balance out, then 6 is the lighter one.

    If {1,2,3} and {4,5,6} balance out, then we know either 7 or 8 is the lighter one. Weigh both of them to find out which one is lighter.

    Anand Guru Answered on 13th July 2015.
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