Hourglasses Puzzle | Puzzles for Interview - Interview Preparation PDF Download

Introduction

Hourglasses are a popular tool for measuring time, but did you know you can use them to solve puzzles too? In this puzzle, you are given two hourglasses, one that runs for 4 minutes and another that runs for 7 minutes. Your task is to measure 9 minutes using only these two hourglasses.

Solution

Step 1: Start both hourglasses at the same time.
Step 2: When the 4-minute hourglass runs out of sand, flip it over to start again.
Step 3: When the 7-minute hourglass has been running for 7 minutes, there will be 3 minutes of sand left in it.
Step 4: When the 4-minute hourglass runs out of sand for the second time, it will have been running for a total of 8 minutes, but there will be one minute of sand left in the 7-minute hourglass.
Step 5: Flip the 7-minute hourglass over so that the sand runs from the other side, leaving 1 minute of sand in the top.
Step 6: When the 7-minute hourglass runs out of sand for the second time, 9 minutes will have passed in total.

Conclusion

By following these steps, you can measure 9 minutes using only a 4-minute and a 7-minute hourglass. This puzzle requires logical thinking and problem-solving skills, making it a fun activity for anyone looking to exercise their brain.