Can anyone please explain to me the concept for cracking this Physics question???

Question

Before I begin, I'd like to let you know that I AM NOT ASKING YOU TO DO MY HOMEWORK. In fact, this isn't even homework. I've got my Physics Exam comin' up and here's a question I've run into that I can't crack...

Please try and explain to me how it works rather than simply giving me the calculation...

Here's the question:

Town A and Town B are connected by regular bus service with a bus leaving in either direction every T minutes. A man cycling at a speed of 20 km/hr in the direction of A to B notices that a bus goes past him every 18 minutes in the direction of his motion, and every 6 minutes in the opposite direction. What is the period T of the bus service and with what speed do the buses ply on the road?

Answer 1

It looks like your problem can be resolved by understanding the concepts of the Doppler effect (which is what you likely need for any upcoming physics test).  The buses move like waves (km/hr) with wavelengths (km) and you are a moving observer.