Rounding problem

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Rounding problem

Postby lemi0901 » Mon Jan 25, 2010 7:18 pm

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.

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Postby stapel_eliz » Tue Jan 26, 2010 12:07 am

There is more than one way of rounding, and sometimes some "industries" (like economics, or a particular course) use methods other than the one(s) you may have learned previously.

What you've outlined is one of the other methods. I've most-commonly run into this method in business and finance. But if your instructor wants to use it in another context, then it's the "right" method in that context (and with that instructor). Don't be surprised if other instructors in this field use other methods. :wink:

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