must be a positive integer, which means  must be a
 positive integer, so z must have factor 31.

 must be a positive integer, which means  must be a
 positive integer, so z must have factor 20.

So the smallest integer that meets those two conditions is 31×20 or 620

Now let's see if 620 meets the third condition

 or  must be a positive integer, and since

 = 4, 620 meets all 3 requirements so we know that 620 is the answer.

Edwin