I need help answering this using Quadratic Equations. Please, also explain in detail what you did, so I KNOW how to do it for next time.

The time required for a train to make a 175 km trip is 1.5 hours less than the time required by bus. The average speed of the bus is 15 km/hr less than that of the train for the same trip. Find the average speed of the train.

Thanks =)