I am a college student, who has been told, consistently, the same rules of rounding decimals since I was in third grade. 17.52 is 18, and 17.49 is 17. Today, my chemistry professor threatened to turn this very basic rule upside down for reasons I cannot comprehend.

His edition of the rules are as follows:

In the instance that 5 is the first non-significant figure in a number, he says to look to the next non-significant figure. No problem. At which point he says the following thing:

If 5 is not followed by anything, round up

If 5 is followed by an odd number, round up

If 5 is followed by an even number or zero, round down.

For example, if we were to round the following numbers to the third significant figure:

17.150 rounds down to 17.1

17.151 rounds up to 17.2

17.152 rounds down to 17.1

And so on.

Does this make a bit of sense to anyone out there? I need to know. If this is correct, and I have been told wrongly the rules of rounding up until this day, I would like to know. An explanation would be helpful too, as to why this rule is true.

Thank you very much.