You have 10 boxes that contains balls from different manufacturers, with each of the ball weighing 10 grams precisely. One of the manufacturer made a mistake and its balls weigh 9 grams each.

You don’t know which box contains the defective balls.

You have been provided with an electronic weighing machine but you are allowed to use it only once.

Number all the boxes from one to ten. Since you can weigh only once, you have to pick balls from each box and weigh them together.

Pick one ball from Box 1, 2 balls from Box 2, 3 balls from Box 3, and so on till, 10 balls from Box 10. Thus after taking balls from each box, you will finally have 55 balls in total.
Put all of them on the weighing machine. It should weigh 55 into 10 equals, 550 grams, if none of the balls were defective.

Say the weight is less by 3 grams. It means 3 of the balls are defective. Since you picked three balls from the box number 3, that particular box has the defective balls.

If box one had defective balls, it will weigh less by one gram as you picked only one ball from it. If box 2 had defective balls, it will weigh less by 2 grams and so on. If it weighs less by 10 grams, Box 10 has the defective balls.

In this way you’ll be able to find out the box with the defective balls.