8 Balls Problem | Puzzles for Interview - Interview Preparation PDF Download

Problem Statement

You are provided with 8 identical-looking balls and a balance scale as a measuring instrument. Out of the eight balls, seven have equal weight, and one ball is defective and weighs less than the others. The task is to identify the defective ball using exactly two measurements on the balance scale.

Solution

To find the defective ball in two measurements, follow these steps:

1. First Measurement:

Divide the 8 balls into 3 groups: Group A with 3 balls, Group B with 3 balls, and Group C with 2 balls. Now, weigh Group A against Group B using the balance scale.

2. Analyzing the First Measurement:

• If the scale balances, it means that the defective ball is in Group C, which has two balls.
• If the scale tips, it means that the defective ball is in the lighter group, either Group A or Group B, both containing 3 balls.

3. Second Measurement:

• If the defective ball is in Group C, simply weigh the two balls against each other. The lighter one is the defective ball.
• If the defective ball is in Group A or Group B (both with 3 balls), take two balls from the lighter group and weigh them against each other. 

If the scale balances, the third ball from the lighter group is the defective one. If the scale tips, the lighter ball from the two being weighed is the defective one.

Conclusion

By following the solution mentioned above, one can easily find the defective ball using only two measurements on the balance scale. This puzzle tests your ability to think logically and your problem-solving skills in a constrained environment.