The answer will be 45 minutes. If you need a explanation, keep reading.
Think of the viewpoint of a passenger on one of the buses. To him, it looks as if the other bus is moving away at a rate of the difference of the two speeds (25 - (-15)) = 40mph. The negative 15 is because the buses are moving in opposite directions. Then, you can see how long it will take the bus that appears to be going 40mph 30 miles. To do this, rewrite 40mph as 1hr/40mi. Then, to cancel the miles and be left with hours, multiply by 30mi. This gives 3/4 hrs, or 45 minutes
I hope this helps! :>
