David's horse

Algebra Level 2

David loves to ride his horse. When he is riding his horse, he travels at a rate of $10$ mi/hr, and when he is walking, he travels at a rate of just $3$ mi/hr. David goes on a $4$ -hour trip, traveling for a while on his horse and walking the rest of the way, and travels $29.5$ miles. For how many minutes was David riding his horse?