Help Need help with a calculation/equation please.

[kyrsyan]

I don't know why but my brain is not wrapping around the math for this right now. I'm working on a homemade version of wet ones. For this attempt I want to mix isopropyl alcohol and witch hazel. It needs to be roughly 60% alcohol to retain antiseptic/antibiotic properties.
The dilution formula is typically .7x = .6 for 70% isopropyl and water. But witch hazel has 14% alcohol. And for some reason my brain just can't create that equation this week and I'm tired of waiting for it to jumpstart.
Help please. I need the equation, not just the answer, because I will probably be trying quite a few mixes over the next few months until I find one that I'm happy with. I want to add colloidal silver. And maybe some herbs that have antibacterial/antiseptic properties.

 

[1911user]

x = volume of 70% isopropyl alcohol
y = volume of 14% alcohol witch hazel

(x+y) 0.6 = 0.7x + 0.14y

reduces to: y = 0.217x

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a more general equation:

v = desired alcohol percentage of final mixed substance
x = volume of substance 1; a = alcohol percentage of sobstance 1
y = volume of substance 2; b = alcohol percentage of sobstance 2
z = volume of substance 3; c = alcohol percentage of sobstance 3

(x + y + z) v = ax + by + zc

 

[kyrsyan]

Thank you! Once I saw it the brain went "oh yeah". Geez. Thank you.