All students who work hard graduate . This is the transitive law for subsets. The set B, which is in both of the first two statements, is not in the conclusion. Let A be the set of all people in this family, let B be the set of all people who can play chess, and let C be the set of all clever people. Therefore, it is not possible to say whether or not there are clever people in the family.
All frogs can swim.
No animal that can swim will drown. No donkey is stupid. All stupid animals can be caught easily. B' is empty . For example, let A be the set of all birds, and let B be the set of all animals.