what is the value of teh voltage between points X and Z?

I assumed that since this was a parallel circuit that the answer is the same as the voltage source or 100V

 

For starters current ONLY flows through a CLOSED loop.

So what is the voltage drop across X and Y first.?

And what is the volt. drop across Y and Z?

You have the right answer but your assumption is not entirely correct.

If X to Z were closed and Y was disconected than you would have a parrallel circuit.