Consider the history of exchanges as if it were described in two separate deals where:

In the first deal, the man buys the goat for $ 70 and sells it again for $ 80, so he gets a profit of $ 10.

In the second deal, the man buys something for $ 90 and sells it again for $ 100 so he gets a profit of $ 10 again.

Conclusion: the name gets a total benefit of $ 10 + $ 10 = $ 20.