Twitter is useless.
I asked a question yesterday. It was
"If I have a spirit which is 38% and one which is 15%, what proportions do I need to mix it to get 20%?"
Reason: I had some blackcurrant gin which was 20% alcohol and I ran out. I had a bottle of gin and a bottle of cassis, 38% and 15% respectively. And I had man flu so I couldn't work it out.
Clearly, you need to put more of the 15% in to get to 20% from 38%.
(38 +15x)/(x+1) =20.
38 + 15x = 20x + 20
38 - 20 = 20x - 15x
18 = 5x
x = 18/5
18 cassis to 5 gins, or 3 and 3/5, or pretty much 7 to 2.
But it is clear from the above that if you substitute H for 38 (high content) and L for 15 (low content), and T for 20 (target content), you get
(H + Lx)/(x+1)=T
H + Lx = Tx + T
H - T = Tx - Lx
H - T = x(T - L)
The proportion is then given by
(H - T)/(T - L)
There. That's 2 minutes of your life that you won't get back.