x+y>=z 
solution is mx+ny=z 
Bézout's identity
z is the greatest common divisor of x and y 

class Solution(object):
    def canMeasureWater(self, x, y, z):
        """
        :type x: int
        :type y: int
        :type z: int
        :rtype: bool
        """
        def gcd(a,b):
            while b: 
                a,b=b,a%b
            return a
        return (z==0)or ((x+y>=z) and (z%gcd(x,y)==0))