These sorts of problems (particularly the ones like ping pong balls on a bus or washing windows in Seattle or how much water flows out of the mouth of the Mississippi river) are often called "Fermi problems" in honor of a 20th century nuclear physicists who used to spring them on grad students during the final oral exam before being awarded a PhD (talk about high-pressure!).

They show:

- a degree of practical knowledge (what are the dimensions of a school bus?)
- a degree of mathematical skill (how do you calculate the volume of a school bus)
- an ability to deal with uncertainty & approximations
- an ability to think on your feet under pressure

I think all of these skills are applicable to most higher level jobs. The answer is not really the goal, but to see the thought process.

For example

- A bus is tall enough for an adult to walk in, so it is ~6' tall
- A good-sized bus seats 60 students in 10 rows, and each row would have to be about 3' from the previous, so 10 x 3 = 30' long inside. (3' might be a little long, but there is also space for the driver & door)
- a 3' wide bench would seat three kids (or two adults) and another 1.5' for the aisle = 7.5'
- A pingpong ball is ~1" in diameter, so each one will have a volume of about 1 in^3
- The volume of the bus is ~70in x 360in x 90in = ~ 2,400,000 cubic inches = 2,400,000 ping pong balls.
- There is will be some space taken up my the seats, but the spheres call also pack a little more densely than cubes, so those factors will approximately cancel

That is off the top of my head. I could also use knowledge about the typical size of semi trailers to estimate the dimensions of the bus. I just googled the size of a ping pong ball and found it is a little bigger than I estimated (1.38"), which would cut my answer by more than half, to about 1,000,000.

This is still twice what the article estimates. I would challenge them to defend their values. As I mentioned earlier - it is the thought process (and general knowledge) that are important - not the exact number.

Tim F