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Puzzle Details

Imagine you have one cup of tea and one cup of coffee.

Place a spoonful of the tea into the coffee and stir well.

Then place a spoonful of the tea/coffee mixture back into the tea.

Does the cup of coffee now contain more tea, or the cup of tea more coffee?

[Ref: ZGED]

Surprisingly, they are actually the same.

After both swaps have taken place, both cups contain the same amount of liquid that they each started with.

So, any tea missing from the cup of tea is now in the cup of coffee.

The same amount of coffee must be now missing from the cup of coffee, and must be in the cup of tea.

This can also be shown using algebra:

If each cup contains 100 units of its respective beverage and a spoon holds 10 units, then at the start we have:

CoffeeCup = 100 Coffee
TeaCup    = 100 Tea

We now transfer a spoonful of tea across to the coffee to give:

CoffeeCup = 100 Coffee + 10 Tea
TeaCup    = 90  Tea

We now transfer a spoonful of the coffee/tea mixture back to the tea cup, this 10 unit spoonful will contain [10 x (100 Coffee + 10 Tea) / 110]. So we will have:

CoffeeCup = 100 Coffee + 10 Tea - [10 x (100 Coffee + 10 Tea) / 110]
TeaCup    = 90  Tea             + [10 x (100 Coffee + 10 Tea) / 110]

If we simplify this we get:

CoffeeCup = (10000 / 110) Coffee + (1000 / 110) Tea
TeaCup    = (10000 / 110) Tea    + (1000 / 110) Coffee

So there is as much coffee in the tea cup as there is tea in the coffee cup!

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