Question

40x/64ydivided by 5x/8y=

Answer 1

40x/64ydivided by 5x/8y would be better written as

40x/64y
------------    which in turn would be clearer if written as
5x/8y

  40x
--------
   64y
==========
    5x
  ------
    8y

Now invert (5x/8y) and multiply (40x/64y) by your result:

  40x      8 y
-------- * ------    Reducing, we get  8/8, or 1.  The answer is 1.
   64y     5x