4 workers can complete a job in 10 hours. Working at the same rate per worker, how long would 5 workers take to complete the same job?

The total work is fixed: $4$ workers for $10$ hours is $4\cdot10=40$ worker-hours. With $5$ workers, each doing the same amount of work per hour, the time is $\frac{40}{5}=8$ hours, choice F. More workers means less time, which is inverse variation. Choice H ($12$) and choice J ($12.5$) increase the time when workers are added, which cannot be right. Choice G ($8.5$) is close but does not follow from the constant-work relationship. The product of workers and time stays $40$.