Conclusion wrong: right one is “Some vain persons are not Professors.”

19. A prudent man shuns hyænas.
No banker is imprudent.
No banker fails to shun hyænas.

Univ. “men”; m = prudent; x = shunning hyænas; y = bankers.

m1x′0 † ym′0x′y0 [Fig. I.

Conclusion right.

20. All wasps are unfriendly;
No puppies are unfriendly.
No puppies are wasps.

Univ. “creatures”; m = friendly; x = wasps; y = puppies.

x1m0 † ym′0x1y0 [Fig. I (α).

Conclusion incomplete: complete one is “Wasps are not puppies”.

21. No Jews are honest;
Some Gentiles are rich.
Some rich people are dishonest.