You can make 48 cubes in the time it takes to freeze one tray (assuming it does not take any longer to freeze multiple trays at once than it takes to freeze one tray). You can then continue to add 72 new cubes for each increment of time it takes to freeze one tray.

Proceed as follows: fill one tray with water, put another tray upside down on top of it, then another full tray, then an empty, etc. This gets you four trays or 48 cubes in the time it takes to freeze one tray.

By using frozen cubes as spacers to hold the trays apart, you can make an additional 72 cubes in the time it takes to freeze one tray. Place two cubes in the opposite corners of six trays, and fill the rest with water. Freeze all six, plus a seventh you put on top, at the same time.

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