Rolf wrote:

> Now, in your head, what is a third of 4m 22cm 5mm ?

If you're asking this, you haven't understood the relationship between the
"metric" system (which is only called so by the "imperials" :) and our
number system. No one working with SI units would even think of giving a
length in that form. It's either 4.225m, 422.5cm or 4225mm (to cite the
more common ways), and it's easy to convert between the three. 

> You see, multiple units is never a good thing for mental math... 

Exactly. That's one beef with imperial units; the conversions between them
don't match the number system you're doing it in, which complicates
matters. I agree with you that a base-12 number system has advantages, but
I challenge you to do the math in your head, using a /real/ base-12 system
:)  I think I stick with the decimal system, at least as long as everybody
else does it.

> even converted to a single unit 4225 / 3 ?

You're talking about building, so generally you wouldn't measure in mm, but
in m. (Maybe in cm, if you're thinking from a woodworking perspective, but
the changes to the following are minimal -- due to the use of a number
system that matches the unit system.) That would then be 4.225m. 4.2m / 3
is easy; that's 1.4 m. If you need the precision and want to add the
remaining 25 mm / 3, you can do that just as easily: 1.4 m + 8 mm = 1.408
m. I think that's a lot less meddling around with numbers than what you
described: two simple divisions and an even simpler addition, without
"having to know" much :)

Gerhard

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