Quantitative Interview Practice Test

The most effective way to identify the defective coin-producing machine with just one weighing is by weighing the coins against a standard weight. This method allows for a direct comparison which reveals any discrepancies in weight that would indicate a problem with one of the machines.

When you weigh the coins against a standard weight, the expected weights of coins from each machine can be calculated based on the known correct weight of a coin. If one machine produces defective coins that are either heavier or lighter than the standard, the comparative weigh-in will show a deviation from the expected total weight, making it possible to pinpoint the source of the defect.

While taking one coin from each machine provides a sample, it does not allow for a comprehensive enough comparison to determine defects unless each machine's output is uniform. Similarly, comparing samples of different sizes or weighing coins from only one machine would not provide a reliable method to detect which machine is faulty, as these approaches lack the necessary breadth to encompass possibilities from all machines effectively.