Suppose we want to round a number to the nearest (one, ten, hundred, thousand, etc). To do so, we look at the (ones, tens, hundreds, thousands, etc) digit, then look one digit to the right of that.
If the digit to the right is greater than or equal to #5#, then we increase the (ones, tens, hundreds, thousands, etc) digit by #1# and set all the digits to the right of it equal to #0#.
If it is less than #5#, we just set all the digits to the right of the (ones, tens, hundreds, thousands, etc) digit to #0# directly.
In the given example, we want to round #1993# to the nearest hundred, so first we look at the hundreds digit:
#1color(red)(9)93#
Then, look at the digit to the right:
#19color(red)(9)3#
This digit is greater than or equal to #5#, so add #1# to the hundreds digit and set all the digits to the right of it equal to #0#:
#1(9+1)00#
#1(10)00#
In this case, we now have #10# hundreds, which is the same as #1# thousand, so add #1# to the thousands digit and set the hundreds digit equal to #0#:
#(1+1)000#
#2000#
and we are done. #1993#, rounded to the nearest hundred, is #2000#.
