B) At least one safety inspector must work more than 40 hours per week

A – Fit-well but non Fact-based

There are 4 inspectors out of 20 direct workers so it seems like cost of inspector can very well be 1 / 5 of total direct worker cost. But a missing piece of data to conclude that is: each person get similar total income.

C – Fit-well but non Fact-based

The mine open 9 hours per day, 7 days per week, and there must be 10 line workers at a time, so it is 630 man hours per week at the line positions. There are 16 line workers, so on average each must work only 39 hours per week. This seems to fit very well with the proposed conclusion: line workers do not work more than 40 hours per week. However, a missing piece of data to conclude that is: every line workers work a same amount of time (if not, there can be some who work over 40 hours, some others work less).

D – Fit-well but non Fact-based

Similar to A, there are more line workers, so it seems like total cost for line workers is more than total cost for safety inspectors. But a missing piece of data needed to conclude that is: cost of each workers is similar.

Only B is a proven TRUE conclusion based on the provided facts. There are 24 x 7 = 168 inspector hours needed in a week, equally 42 hours per week per inspector. So there must be one who work more than 40 hours.