code: function checkRateRange(num) {
      var numString = num.substring(0, num.length - 1);
      var parseNum = parseInt(numString);
      if (parseNum < 20) {
          throw new Error("The minimum rate is twenty percentage. Received: " + parseNum);
      }
  }
docs: This method ensures that the value of the rate must be equal or great than 20%
  @param num is the value of rate