What Is Proof by Contrapositive?

Billy goes to the beach one day with his little brother, Libby.

The following statements are true:

• If Libby is wading in the water, then Billy will wade in the water, too.

• Libby will buy a scoop of ice cream if he spots an ice cream truck.

• If Libby creates a sand castle, the sand castle will be washed away.

If the above statements are true, then which of the following statement(s) is/are also true?

(assume that ONLY the original statement and the contrapositive is true)