input: Write a function to determine the commission rate based on gross sales.
  assert determine_commission_rate(15000) == 0.1
  assert determine_commission_rate(7000) == 0.07
  assert determine_commission_rate(3000) == 0.05
output: def determine_commission_rate(gross_sales):
      if gross_sales >= 10000:
          return 0.1
      elif gross_sales >= 5000:
          return 0.07
      else:
          return 0.05