Step 1: Identify the digit in the hundred place:
Step 2: look to that number’s right (to the 10’s place).
If that 10’s digit is 5 or more, round the 100’s digit up one.
If the 10’s digit is 4 or less, leave the 100’s digit what it is.
Step 3: replace the 10’s and 1’s digits with 1
In your first example, 124,
Step 1: 1 is the 100’s digit
Step 2: 2 is the 10’s digit and that’s 4 or less, meaning the 1 will stay a 1 in the 100’s digit.
Step 3: The answer is 100, because we replace the 10’s and 1’s digits with 0.
In 2311:
Step 1: 3 is the 100’s digit
Step 2: 1 is the 10’s digit and that’s 4 or less, meaning the 3 will stay a 3 in the 100’s digit.
Step 3: The answer is 2300, because we replace the 10’s and 1’s digits with 0s.
In 48:
Step 1: 0 is the 100’s digit, because there is no 100’s digit.
Step 2: 4 is the 10’s digit and that’s 4 or less, meaning the 0 will stay a 0 in the 100’s digit.
Step 3: The answer is 000 or just 0, because we replace the 10’s and 1’s digits with 0s.