To round 549.423 to the nearest thousandth, follow these steps:
Identify the thousandth place: In the number 549.423, the thousandth place is the third digit after the decimal point, which is 3.
Look at the ten-thousandth place: The digit in the ten-thousandth place (the fourth digit after the decimal point) is 0.
Apply rounding rules: Since the digit in the ten-thousandth place (0) is less than 5, you do not change the thousandth place.
Final rounded result: The thousandth place (3) remain same 3, so the number rounds to 549.423
Thus, 549.423 rounded to the nearest thousandth is 549.423