firstname.lastname@example.org writes: >Now balloons are listed as "12 inches". I am guessing that an >inflated balloon, and empty space around it when it is put in a large >room with other balloons would equal maybe 1.25 cubic feet.
If the balloon has a one-foot diameter, that doesn't mean it occupies one cubic foot when inflated! That would be true if the balloon were cubical in shape, rather than spherical.
The volume of a cube is 4/3 pi r^3. Your radius is 1/2 foot, so your volume is pi/6 which is about 0.524 cubic feet.
You're right that there will be some empty air space (depending on how tightly you cram the balloons in -- you could *force* them to be more or less cubical (and smaller than advertised) if you really try hard -- but still, you'll need more balloons, not fewer, than you would if each balloon occupied exactly one cubic foot.