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Box with Defective Balls

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Here is a situation. You have 10 boxes that contains balls with each of the ball weighing 10 grams precisely. Now among the boxes, one of the box comprises of defective balls with each defective ball weighing 9 grams. You have been provided with an electronic weighing machine but you are allowed to use it only once.

Can you find out which box contains defective balls?

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ravi Expert Asked on 19th July 2015 in Microsoft Interview Puzzles.
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    • 1 Answer(s)

    The question can practically disarm you. But we promise by the time you read the solution, you will find that it was way easy than you were thinking.

    Let us name the boxes with a number – Box1, Box 2…, Box10. Now you must be familiar with the weights of the balls precisely. By saying that we are not implying that you will have to take one ball from every box and judge the weight of every ball.

    What you have to do is pick one ball from Box 1, 2 balls from Box 2, 3 balls from Box 3, …., 10 balls from Box 10. Thus after taking balls from every box in such a manner, you will finally have 55 balls in total. There comes the time to use the weighing machine.

    If all the balls were weighing adequate, the combined weight should be 55 x 10 grams = 550 grams. But since one of the box has defected balls, the weight will be less than that. Here is the tricky part and explains why we took different number of balls from each box.

    If the total weight is less than 1 gram, then the defective box is the Box 1 since we took 1 ball from that box. If the Box 2 is defective, the total weight will be less than 2 grams. In similar fashion you can identify the defective box by analyzing the total weight that is calculated by the weighing machine.

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    John123 Expert Answered on 19th July 2015.
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