To round 3.991 to the nearest hundredth, follow these steps:
Identify the hundredth place: In the number 3.991, the hundredth place is the second digit after the decimal point, which is 9.
Look at the thousandth place: The digit in the thousandth place (the third digit after the decimal point) is 1.
Apply rounding rules: Since the digit in the thousandth place (1) is less than 5, you do not change the hundredth place.
Final rounded result: The hundredth place (9) remain same 9, so the number rounds to 3.99
Thus, 3.991 rounded to the nearest hundredth is 3.99