Blending beers, calculate FG?

[rockdemon]

I know that someone here told me how to calculate this but i cannot find the thread.
Its probably just basic math, but im really bad at math.
I want to increase the FG on a superdry beer by blending it with a beer with higher FG.

I have X liters of beer with X FG. How many liters of X FG beer does it take to reach desired FG.

With made up values:
10 Liter of 1.005 beer. how much 1.020 beer does it take to make a 1.012 beer?

the 2 beers have pretty much the same OG.

 

[freisste]

It would be a PITA to explain this on a forum (especially on my iPhone). This is what I have come up with:

V_A = V_O*((target gravity-original gravity)/(added gravity-target gravity))

Said in words, the volume to add is equal to the original volume times the difference in current beer gravity and target gravity divided by the difference in the added beer and target gravity.

 

[CrypticIntent]

It's a simple weighted average problem

V1*G1 + V2*G2 = (V1+V2)*G3

We only need to know V2 so:
V2 = V1 * { (G3 - G1) / (G2 - G3) }

So for your made up values

V2 = 10L * { (1.012 - 1.005) / (1.020 - 1.012) }
= 10L * { .007/.008 }
= 8.75 Litres