To round 121.58 to the nearest thousandth, follow these steps:
Identify the thousandth place: In the number 121.58, the thousandth place is the third digit after the decimal point, which is 0.
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 (0) remain same 8, so the number rounds to 121.58
Thus, 121.58 rounded to the nearest thousandth is 121.58