Solution for #15

Alternate Solution #1

Ask one of the men what the other man would answer to the question, "Is the left road the correct road?" Then assume the answer you are given is false and act on that knowledge.

If the man you ask is the liar, he'll incorrectly give you the truthful man's answer. If the man you ask is the truthful man, he'll correctly give you the liar's wrong answer.

Alternate Solution #2

Ask one of the men, "If I had asked you which path was the correct path ten minutes ago, what would you have said?" Regardless of which man is asked this question, the answer will be the correct path.