input: Write a python function to calculate simple interest and return the final amount.
  ```python
  def calculate_simple_interest(principal, rate, time):
      return principal * (1 + rate * time)
  
  assert calculate_simple_interest(20000, 0.10, 2) == 24000
  assert calculate_simple_interest(15000, 0.05, 3) == 17250
  assert calculate_simple_interest(10000, 0.07, 1) == 10700
output: def calculate_simple_interest(principal, rate, time):
      return principal * (1 + rate * time)