Final answer:
Twelve men can install 20 power line poles in four hours. After one hour, when five men leave the job, it will take the remaining men approximately 0.57 hours to complete the job.
Step-by-step explanation:
To solve this problem, we need to determine the rate at which the men work. If twelve men can install 20 poles in four hours, then their rate of work is 20 poles / 4 hours = 5 poles per hour.
After one hour, five men leave the job, so there are now seven men remaining. The remaining men's combined rate of work is 7 men * 5 poles per hour = 35 poles per hour. Therefore, it will take the remaining men 20 poles / 35 poles per hour = 0.57 hours to complete the job.