Testing validity

Identify and explain the validity of the following arguments:

1. The car was running out of petrol, so it must have stopped.
2. Joe can travel by train at half price because he has a student's rail card.
3. All intelligent people are short-sighted. I am very short-sighted, so I must be a genius.
4. Only if he is of age is the accused liable to a fine if he is guilty. Since he is only sixteen, he cannot be liable to a fine, although his guilt is not in doubt. For at sixteen years old he is not yet of age.