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
-60 + 70 = 10
-80 + 90 = 10
The man made $10 with each transaction, therefore his profit adds up to $20.
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