Provide your best answer for the following math problem. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The problem is: two cars start at the same time from opposite ends of a highway that is 105 miles long . one car is riding at 15 mph and the second car is riding at 20 mph . how long after they begin will they meet ?
3
5.25 hours or 315 minutes.