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.
Did you approach it differently? Let us know in the comments, and SHARE this problem to test your friends.