# Re: [MBZ] touchups

```On Fri, 11 Aug 2017 12:51:27 -0400 Dan--- via Mercedes
<mercedes@okiebenz.com> wrote:```
```
> It is, however, it's a common thing used by nearly everyone in the
> detailing community.  A lot less aggressive than solvents, too.

Alcohol is a solvent ...

> You want a 50-50 mix of water to alcohol if possible, so you'll have to
> do some math if you're using typical 70% alcohol you find at the
> pharmacy.  I usually go for the highest concentration I can get,
> typically in the 90% range, which makes it a little easier to do the
> math.

With some trial and error, I figured out how to calculate it (I'll leave
the details of the derivation out):

Volume of water
------------------------- =  2*p - 1
Volume of alcohol mixture

where p = fraction of alcohol in alcohol
mixture (for 70%, p = 0.7)

So, for 70% alcohol mixture,

Volume of water
------------------------- =  2*0.7 - 1 = 0.4
Volume of alcohol mixture

Thus, Volume of water = 0.4 Volume of alcohol mixture

For 91% alcohol mixture,

Volume of water
------------------------- =  2*0.91 - 1 = 0.82
Volume of alcohol mixture

Thus, Volume of water = 0.82 Volume of alcohol mixture

Checking: For 100 ml (could also be ounces) 70% alcohol mixture

Alcohol = 70 ml
Water   = 30 ml (from mixture) + 40 ml (added to mixture)
= 70 ml

Checking: For 100 ml (could also be ounces) 91% alcohol mixture

Alcohol = 91 ml
Water   = 9 ml (from mixture) + 82 ml (added to mixture)
= 91 ml

Craig

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```