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It went like this:
(For simplicity's sake, presume 1 tsp = 1/100th of the 4-oz volume)
1 tsp milk removed from milk glass, added to tea glass =
tea glass = 100 parts tea + 1 part milk; milk glass = 100 parts milk - 1 part milk = 99 parts milk
Volume of tea glass = 101 parts Volume of milk glass = 99 parts
.
1 tsp tea/milk mixture (99% tea, 1% milk) removed from tea glass, added to milk glass =
tea glass = 99.0099... parts tea + 0.990099... part milk; milk glass = 99.0099... parts milk + 0.990099... part tea
Simplified: (99.01 parts tea + 0.99 part milk) in tea glass; (99.01 parts milk + 0.99 part tea) in milk glass
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Bottom Line - you end up with:
1 glass with 99% tea 1% milk, 1 glass with 1% tea 99% milk ---- IOW
the same milk-in-tea, as tea-in-milk.
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The number of ounces in the glasses is irrelevant, so long as they are the same. (The original volume is 4 ounces each.)
They could be 20 oz or 3,419 ounce glasses, no difference. So long as they're the same volume.
The puzzle says,
"8 ounce glasses... half full..." just to distract you and to make you think of half volumes.
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The total volume in each glass, regardless of its size, is unchanged by the transfer.
The same volume is removed, transferred, removed and transferred back.
So the first transfered volume is canceled by the second.
And the volume of milk transferred in the first teaspoon is exactly canceled with the volume of tea/milk transferred back.
If this process were repeated many times, eventually both glasses would end up with 50/50 tea and milk, with the same volume.