Six identical workers, working independently at a constant rate, complete one job in 10 hours. How long would 15 such workers take to complete the same job?

Explanation

The job requires 6 times 10, or 60 worker-hours, so 15 workers need 60 divided by 15, which is 4 hours. The 25-hour result applies direct scaling to an inverse situation, even though adding workers must reduce the completion time.