Passage Summary: John has a five-day work week consisting of four days at an insurance office and Fridays at a forge; since last week was a normal week, the speaker concludes he was at the insurance office from Monday to Thursday.

Conclusion: John definitely worked at the insurance company on Monday, Tuesday, Wednesday, and Thursday of last week.

Reasoning: John works five days a week (unless it's a holiday or vacation), with four days at an insurance firm and Fridays as a blacksmith.

Analysis: The argument takes for granted that John's five-day work week is strictly Monday through Friday. While we know he works four days at one place and Fridays at another, nothing in the premises prevents him from working, say, Tuesday through Saturday. To make the conclusion follow, we must assume his work week does not include any days other than Monday through Friday. This is the classic 'missing link' between a total number of days and specific calendar days.