Question 402205
The jar's volume is 9*9*12 = 972 cubic inches
If the gumballs are stacked with their centers on a grid, one above the other, each will occupy (3/4)^3 cubic inches = 27/64.
972/(27/64) = 2304 gumballs.
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Stacking is more complex, tho, and the gumballs will stack such that their centers are not vertical.  The jar will hold more than 2304, maybe 2500 or 2600.