This type of questions are basically Preposterous questions. You should ask some basic questions regarding size of the ball, is the plane empty or occupied with people, shall i consider the fuel tanks and other parts or just in side the cabin .Then proceed with taking the plane as a combination of certain geometric figures guesstimating its dimensions. Finally in certain volume the max number of balls can be filled when arranged with second layer fitting in the empty space created by the first layer, find for certain volume and extend to the whole volume of the plane. You should not equate (volume of plane/volume of ball =No.of balls), this approach is wrong , you should have the way of measuring the volume wasted or effective area. Thanks&Regards