Hi Alan,

Let's draw out in the number line to understand this better,



This is a typical "COULD-BE" question in GMAT. So Plug-In

Could be - Is it true for anyone set of values.

So all you need to do, pick one set of values check whether it matches anyone answer choice.

Let x =8, w = 10 then z = 12 and y = 11.

So the ratio of (w-y)/(z-x) =(- ¼)

Here the key is "w is the midpoint of x and z".

So the answer is B.

You need not worry about whether the answer will change if we take the negative values.

Remember it is "could be" question so if one option works so then that's the answer.

I hope this is clear.