How do you round off the whole number?

Whatever place you are rounding off to, look to the digit to the right of it. If it is 4 or less, the digit you are rounding stays the same and everything to the right of it becomes a zero. If the digit to the right of it is 5 or greater, the number you are rounding becomes one greater and everything to the right of it becomes a zero.

For example, if I am rounding to the nearest 100 in each of these.

12348 becomes 12300

74862 becomes 74900