input: Write a function to reduce an array by its greatest common divisor (GCD).
  assert reduce_by_gcd([30, 20, 45])==[2, 3, 5]
  assert reduce_by_gcd([12, 15, 21])==[4, 5, 7]
  assert reduce_by_gcd([100, 80, 120])==[5, 4, 6]
output: def reduce_by_gcd(arr):
      from math import gcd
      from functools import reduce
  
      def find_gcd_of_list(lst):
          return reduce(gcd, lst)
  
      gcd_of_arr = find_gcd_of_list(arr)
      return [x // gcd_of_arr for x in arr]