hmm... I'm getting a different answer...
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x = number of passangers with roundtrip tickets
y = number of passengers without roundtrip tickets
so total number of passangers = x + y
question: x/(x+y) ?
"60% of who have round-tickets do not put their cars on the ship board"
this means, 40% of those who have roundtrip tickets do put their cars on the ship.
40% of x = those with round tickets AND put their cars on the ship
and it is given that "For passengers in a ship, 20% have round-tickets and put their cars on the ship board":
20% of (x + y) = those who have round tickets AND put their cars on the ship.
so, we can equate both statements:
40% of x = 20% of (x+y)
0.4x = 0.2(x+y)
x/(x+y) = 0.2/0.4
x/(x+y) = 0.5
IMHO 50%.
-BM-
