Problem 6.4. Alexei, Boris, Benjamin, and Grigory are suspected of robbing a bank. The police managed to find out the following:

- if Grigory is innocent, then Boris is guilty, and Alexei is innocent;
- if Benjamin is guilty, then Alexei and Boris are innocent;
- if Grigory is guilty, then Boris is also guilty;
- if Boris is guilty, then one of the two - Alexei and Benjamin - is definitely guilty. Mark those who participated in the robbery.
Answer: Alexei, Boris, and Grigory participated in the robbery.

Solution. From the first and third statements, it follows that Boris is definitely guilty. Then, from the second statement, it follows that Benjamin cannot be guilty. From the fourth statement, it follows that one of the two - Alexei or Benjamin - is guilty, i.e., Alexei is guilty. Then, from the first statement, it follows that Grigory cannot be innocent, i.e., Grigory is guilty.