Here are three helpful hints if you are trying to help somebody to round numbers:
1. When the first digit dropped is less than 5, do not change the preceding digit. E.g., 2.324 rounded off to the nearest hundredth is 2.32.
2. When the first digit dropped is greater than 5, or 5 followed by one or more non-zero digits, increase the preceding digit by one.
E.g., 2.316 and 2.3151 are both 2.32 when rounded off to the nearest hundredth.
3. When the first digit dropped is 5 and there are no digits following or the digits following are zeros, make the preceding digit even (i.e. round off to the nearest even digit). E.g., 2.315 and 2.325 are both 2.32 when rounded off to the nearest hundredth.
Note: The rationale for the third rule is that approximately half of the time the number will be rounded up and the other half of the time it will be rounded down.