3. At Least One

If A is not selected then, then B must be chosen. You can then take the contrapositive: If B is not selected, then A must do it.

Now we’re going to introduce a new concept: combining conditionals. See this example (shown in the next video):

Premise #1

If Matt is NOT in the Office, then Tim is:

~M T

Premise #2

Contrapositive of #1: If Tim is NOT in the Office, then Matt is.

~T M


At least Tim or Matt must be at the office. Note: There’s nothing ruling out T and M being together.

Either you run the day or the day runs you.

RTD (run the day) DRY (day runs you)

Contrapositive:~ DRY RTD

  • Now that you can combine statements, we can do more complex deductions, such as If and Only If.
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