If a number is a natural number, then it is a whole number.

If a number is a whole number, then it is a natural number.

If I study, then I will do well on my Geometry test.

If I did not do well on my Geometry test, then I did not study.

If I don't set my alarm clock, then I will be late for work.

If I set my alarm clock, then I will not be late for work.

If you purchase two bags of chips, then you will get one bag free.

If you did not get one bag for free, then you did not purchase two bags of chips.

If you live in Denver, then you live in Colorado.

If you live in Colorado, then you live in Denver.