When you perform any arithmetic operation and round off the last digit, this is a bit precise but there are conditions in using the rounding off method. You round off only at the digit nearest the decimal point. For example,
2.21 + 1.175 = 3.385
round off 3.385, it becomes 3.39
The 9 there is the digit which has the same decimal place of 2.21. This is where you should round off your answer.