Question 202651
This a really poor question. It is quite possible that a job that can be done by 6 people, cannot be done by 4 people no matter what the job is. Also, there is nothing to let you know if the 4 person group is subset of the 6. So perhaps the skills held by the 6 person group are not skills the 4 person group has.

That said, I am sure the intent is as follows:
The job requites 6*8= 48 person/hours
If there are 4 people working, the 48/4 = 12. So the group of 4 will take 12 hours to complete the job.