Alice and her brother Bob decide to ride bicycles from their house to a park, which is 3.90 miles away. At 9:00 A.M., Alice begins riding toward the park, at a speed of 10 miles per hour. At 9:10 A.M., Bob starts and heads toward the park, but at a faster speed of 15 miles per hour. How long does it take for Bob to reach the park after Alice?