To round 21,498.2536 to the nearest thousandth, you follow these steps:

1. Identify the digit in the thousandth place: The digit in the thousandth place is 3.
2. Look at the digit in the next place to the right (the ten-thousandth place): The digit here is 6.
3. Apply the rounding rule: If the digit in the next place (6) is 5 or greater, you round up the thousandth place digit by 1.

Since the digit in the ten-thousandth place is 6, you round up the 3 in the thousandth place by 1.

So, 21,498.2536 rounded to the nearest thousandth is 21,498.254.

Do you want more details or have any questions?

Here are 5 related questions you might find interesting:

1. How do you round a number to the nearest hundredth?
2. What are the general rules for rounding numbers?
3. Can you explain how to round a decimal to the nearest integer?
4. How would you round 21,498.2536 to the nearest hundred?
5. What happens when you round a number with 5 in the decimal place you're considering?

Tip: When rounding, if the digit you're considering is 5 or more, always round up. If it's less than 5, round down.