input: Write a function to evaluate a given mathematical expression string.
  assert evaluate('2*3+4/5') == 6.8
  assert evaluate('10+20-5*2') == 20.0
  assert evaluate('100/10+3*2') == 16.0
  assert evaluate('8*2/4') == 4.0
  assert evaluate('7-3+2*5') == 14.0
output: def evaluate(expression: str) -> float:
      return eval(expression)