Given only the information in the post it seems prudent to assume that
both A and B are roughly equivelant in skill level, and that neither
have additional research on the matter (why bother comparing the
efforts of A and B if you're allowed to fudge in that B is a better
programmer and so knows he can do it faster, or you fudge in that B
already knows of a partial solution and so knows he can do it faster
with that as a basis). All we know is that both heard and accepted
the assignment, that A jumped right into research and design and
ignored the possibility of pre-existing solutions and wrote it in
three days. B blew it off completely until Monday without looking at
anything. Presuming relatively equal skill sets it would take B
roughly three days to do it on his own just as for A. B banked
everything on finding a suitable existing solution to modify at the
last minute. A fairly sloppy attitude in my opinion, and something
that is likely to be harder to correct than teaching A the value of
prior art.