Google Foobar challenge: Rabbit Test Subjects
For one thing, the assignment was to define a function answer(x, y), not answer(y, x).  You could also put the built-in function sum() to use.  I believe the key point is rounded to the nearest whole number.  int() always goes down.  That is 4.9 results in 4, but rounding would result in 5.  You can use the built-in function round().  In Python3 it returns an integer, but in Python2 it returns a float.  To take care of both cases, convert the rounded version to an int:

def answer(x, y):
    return int(round((1-(sum(y)/sum(x))) * 100))