Working at the same rate, 4 workers can paint a fence in 10 hours. How many hours would it take 8 workers, working at that same rate, to…

The total work is fixed, so the number of workers and the time are inversely proportional. Four workers for 10 hours is $4 \times 10 = 40$ worker-hours of work. With 8 workers, the time is $\frac{40}{8} = 5$ hours. Doubling the crew halves the time. A common wrong answer is 20, which comes from multiplying instead of dividing, treating the relationship as direct. Another is 14 or 6, which result from adding or subtracting the change in workers (4) to or from 10. Work problems are about the product of workers and time staying constant.