William Sommerwerck wrote:
> cm to inches: multiply by 4, then divide by 10. 30cm is 12" -- the diameter
of an LP.
cm to inches: divide by 2.54 to get a much more accurate result.
-- Nick T.
Oh, how I can feel the temptation to slip back into my "bite the idiot's head
off" mode. I shall try to refrain.
Most people don't understand the concept of rounding off. You see this
ignorance on food packages, where a whole number of ounces becomes a gram
measurement with two digits following the decimal point!
The person who designed the package didn't understand that the calculated
number of grams cannot be more accurate than the accuracy with which the weight
is measured in ounces. 32 ounces can be between 31.5 and 32.5 ounces * -- so
specifying the number of grams to even one decimal place is pointless and
meaningless. **
You see the same sort of thing on eBay. A bid price in British pounds is
converted to the American penny using the exact current conversion rate -- and
the result is labelled "approximate"! Well, a price to the penny is _not_
approximate. You can't have it both ways.
There are 2.54 cm to one inch, so you divide centimeters by 2.54 to get inches.
But that requires a calculator. Dividing by 2.5 -- that is, multiplying by
four, then dividing by ten -- can be done in the head. Using 2.54 does _not_
give "a much more accurate result" -- the difference between 2.5 and 2.54 is
less than 2%.
Too many people reach for their calculator when a quick mental approximation
would give a more-useful result.
* In practice, the FTC requires packages to contain at least the specified net
weight -- never less. But that has nothing to do with the principles involved
in rounding off measurements..
** If the weight were given as 32.0 ounces, then it _might_ be reasonable to
have one decimal place in the conversion.
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