input: Write a python function to calculate the future value of an investment. 
  assert future_value(10000, 10, 5) == 16105.1 
  assert future_value(5000, 5, 10) == 8144.47 
  assert future_value(1500, 7.5, 3) == 1861.91
output: def future_value(principal, rate, years):
      return round(principal * (1 + rate / 100) ** years, 2)