1) The average overtime per month in the manufacturing division of Haglut is 14 hours
2) Most employees of Haglut work in the manufacturing division
3) The average overtime worked by any employee does not fluctuate much from month to month.
The conclusion of the argument is: therefore, each month, most employees of Haglut work at least some overtime.
Now, how could this conclusion possibly be not true?
What if just a few employees--NOT most--in the manufacturing division of Haglut work overtime. BUT these few employees work SO MUCH overtime that they raise the average of ALL employees in the manufacturing division to 14 hours, even though many employees do no work overtime at all.
So it's possible a minority of employees in the manufacturing division bring the average up to 14 hours even though many employees do not work overtime. In addition, we don' know whether non-manufacturing employees work any overtime.
So we cannot say for certain that MOST employees of Haglut must work some overtime.
1 Explanation