I was actually asked a similar question, but with ping pong balls.

The way I went about it (and it seemed to have worked):

• First, i inquired about the Jumbo Jet and, more specifically, i asked if it was compepletely empty or if it was filled with seats and other furniture.
• In case teh interviewer tells you that the airplain is empty, simply calculate the volume of the golf ball, the volume of the different parts of the plane (e.g. body, tail, etc) and then divide the volume of the plane by the volume of the golf ball. Conclude by reducing the number by 5% since it would be impossible for all the balls to perfectly fit in the airplane, hence there is still a certain amount of free space.
• in case the interviewer tells you that the plane is completely furnished, you will have to approximate a capacity of free space (i'd say 60%).
  • in this scenario you should also consider the different between the cabin and the luggage storage on the plane (different capacities)

Hope this helps you