SOLUTION: Prove that the following argument is not valid. "If it rains, crops will be good" if it did not rain, therefore, crops were not good.

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I haven't taken formal logic, but it is possible that crops will be good even if it doesn't rain. This is like saying "If a quadrilateral is a square, it is a rectangle." However, the obverse "If a quadrilateral is a rectangle, it is a square" is not always true. It is true if and only if the relation is one-to-one.

However, the contrapositive is always true and equivalent to the original statement: "If crops are not good, it did not rain."