Pred, I don't think so.
First bloke (a) loads 1/2 a plane an hour
second bloke (b) loads 1/3 of a plane an hour
third bloke (c) loads 1/4 of a plane an hour.
Sooooo.
Lowest common denominator is 12
a = 6/12 per hour
b = 4/12 per hour
c = 3/12 per hour
So in one hour we have 13/12 planes loaded
Now, an hour divided by 13 = 4 minutes 37 seconds
and we only need 12 of those, so
It takes them, working as a team, assuming that each works at his normal rate, 55 minutes and 24 seconds.