input: Write a python function to calculate simple interest given principal, rate, and time.
  assert simple_interest(1000, 5, 2) == 100.0
  assert simple_interest(1500, 4.5, 3) == 202.5
  assert simple_interest(2000, 3, 1) == 60.0
output: def simple_interest(principal, rate, time):
      return (principal * rate * time) / 100