Solution to 4.

4. It is necessary for the post office to deliver a letter that the letter have proper postage.

Doing the groupings first, we get:

It is necessary for (the post office to deliver a letter) that (the letter have proper postage).

The two simple statements are:

P = The post office will deliver a letter.

L = The letter has proper postage.

We have a statement involving necessary conditions, and so we need to identify which statement is the necessary condition. That one will be the consequent of the conditional. The necesary condition is that the letter have proper postage (see the text for an explantion of how to identify necessary and sufficient conditions). So we get: