Add 50, then set the 10s digit and all to the right of it to zero. Examples 153 rounded to the nearest 100 is computed like this. 153 + 50 = 203 203 with the bottom 2 digits set to zero is 200. 19645 rounded to the nearest 100 is computed like this. 19645 + 50 = 19695 19695 with the two bottom digits set to zero is 19600.
You will be told the rule is this. Look at the next digit to the right of the hundreds digit and if it is 5 or more, increase the hundreds digit. Otherwise, don't change the hundreds digit. Set all digits to the right of the hundreds digit to zero. This is how you round to the nearest hundred. You can pick any other place value to round to, and the algorithm is basically the same. (If the digit to the right is 5 or more, increase the one you are rounding to.) The method of addition as indicated above does the same thing. It is also suitable for a calculator that can find the integer part of a number. On a calculator, you would divide by 100, add 0.5, find the integer part of that sum, then multiply the result by 100.
You will be told the rule is this. Look at the next digit to the right of the hundreds digit and if it is 5 or more, increase the hundreds digit. Otherwise, don't change the hundreds digit. Set all digits to the right of the hundreds digit to zero. This is how you round to the nearest hundred. You can pick any other place value to round to, and the algorithm is basically the same. (If the digit to the right is 5 or more, increase the one you are rounding to.) The method of addition as indicated above does the same thing. It is also suitable for a calculator that can find the integer part of a number. On a calculator, you would divide by 100, add 0.5, find the integer part of that sum, then multiply the result by 100.