Try To Solve This Impossible Problem From Across History

If a man buys a horse for $60, sells it for $70, buys it back for $80,

and sells it again for $90,

the trick is to approach each transaction as separate:

-60 + 70 = 10

-80 + 90 = 10

The man made $10 with each transaction,

therefore his profit adds up to $20.
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