How Buying and Selling the Same Cow Twice Makes a Profit

 Conclusion

Although you bought and sold the same cow twice, each transaction is separate. Each one made a profit of €200, giving you a total profit of €400.

 Lesson:
Math riddles like this teach us the value of breaking down problems logically and step by step. Next time you hear a tricky riddle, remember:
Math + Logic =  Profit