Weird ASME rounding rule regarding fractional decimals.

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Jae
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I'm taking an engineering graphics class and I recently learned this rule which states:

"When the digit next beyond the last place to be retained is 5, and there are no digits beyond this 5, or only zeros, increase by 1 the digit in the last place retained if it is odd, leave the digit unchanged if it is even. Increase by 1 the digit in the last place retained, if there are digits beyond this 5."

It just sounds too stupid to be true. What's the purpose of such a rule?
 

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The 5 will be lost.
When you do not have the information needed to decide whether to round up or down, then you round to even.
15.5 will round to 16.
16.5 will round to 16.
17.5 will round to 18.
18.5 will round to 18.

The rounding method does not change the average.