Only a genius can crack this old math problem in less than 20 seconds

The first problem is from a 1984 IQ Test. Ready? 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.