To round 706.421 to the nearest thousandth, follow these steps:
Identify the thousandth place: In the number 706.421, the thousandth place is the third digit after the decimal point, which is 1.
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 (1) remain same 1, so the number rounds to 706.421
Thus, 706.421 rounded to the nearest thousandth is 706.421