Chapter 2 - Reasoning and Proof - 2-2 Conditional Statements - Apply What You've Learned - Page 95: b

I is the converse of II because the hypothesis and conclusion are swapped. Inverse: If a, b, c and d do not form a 2-by-2 calendar square, then $a+d\ne b+c$. Contrapositive: If $a+d\ne b+c$, then a, b, c and d do not form a 2-by-2 calendar square.

The inverse negates the hypothesis and conclusion. The contrapositive both negates and swaps the hypothesis and conclusion.