Hi guys! I'm kind of struggling to put this into algebraic terms, I know the answer but need to show working and just can not get my head around it, if anyone would be so kind and help me out by giving the working and an explanation as to how you got there it would be greatly appreciated. Here is the question:

Two walkers are 15 km apart and they start walking toward each other. One walks twice as fast as other. If they meet 2 hours later at what average speed are they each walking?