Great answers everyone! I really liked some of the ideas there.
I espeically liked:
Lush: Went the route of using volume to calculate.
Banquo: Added to that by using water to calculate these volumes.
Devon Davidson: Went the extra mile to subtract the displacement of the bus itself, leaving only the SPACE in the bus.
But I think the answer that really deserves a gold star is D4's:
"I made a really long comment, it was longer than your post. I'm shortening it now: I figured about 700 golf balls could take up the matter I take up (just a guess) and worked around that, filled up the bus in layed down me's in a bus of say 22 seats, included the drivers seat plus the extra room on the bus and figured the number inclined towards 300k golf balls. That seems wrong, so I think I underestimated the amount of GB's, but it would still be my initial guess"
Now I'm curious to hear the extra long answer you originally had!
Anyway, sorry if I left your name out but everyone did very well and came up with some interesting methodologies (remember, it's not the final answer I'm interested in; it's the methodology!)
Here's one from Vault.com, which specializes in business interviews, that totally blew me away:
Let's assume that the volume of a ping-pong ball is three cubic inches. Now let's assume that all the seats in the plane are removed. We'll say the average person is six feet high, one foot wide and one foot deep. That's 6 cubic feet, or 10,368 cubic inches. (One cubic foot is 12x12x12 inches, or 1,728 cubic inches.)
Okay, so a 747 has about 400 seats in it, excluding the galleys, lavatories, and aisles on the lower deck and about 25 seats on the upper deck. Let's assume there are three galleys, 14 lavatories, and three aisles (two on the lower deck and one on the upper deck), and that the space occupied by the galleys is a six-person equivalent, by the lavatories is a two-person equivalent, and the aisles are a 50-person equivalent on the lower deck and a 20-person equivalent on the upper deck. That's an additional 18, 28, and 120 person-volumes for the remaining space. We won't include the cockpit since someone has to fly the plane. So there are about 600 person-equivalents available. (You would be rounding a bit to make your life easier, since the actual number is 591 person equivalents.)
In addition to the human volume, we have to take into account all the cargo and extra space - the belly holds, the overhead luggage compartments, and the space over the passengers' head. Let's assume the plane holds four times the amount of extra space as it does people, so that would mean extra space is 2,400 person-equivalents in volume. (Obviously, this assumption is the most important factor in this guesstimate. Remember that it's not important that this assumption be correct, just that you know the assumption should be made.)
Therefore, in total we have 3,000 (or 600 + 2,400) person-equivalents in volume available. Three thousand x 10,368 cubic inches means we have 31,104,000 cubic inches of space available. At three cubic inches per ball, a 747 could hold 10,368,000 balls. However, spheres do not fit perfectly together. Eliminate a certain percentage - spheres cover only about 70 percent of a cube when packed - and cut your answer to 7,257,600 balls.
The last, extra step there of reducing 30% is what really makes this answer stand out; taking a largely vague estimate and making peculiar adjustments as that to "fine tune" it. Anyway, back to our regular scheduled programming:
A bus full of people travels from Vancouver to Burnaby. No one gets off the bus for the entire duration of the ride. But when it reaches its destination, not a single person is left on the bus. How come?