Bottle A has 12*0.80 = 9.6 ml of pure alcohol
Bottle B has 8*0.40 = 3.2 ml of pure alcohol
Combined, we have 9.6+3.2 = 12.8 ml of pure alcohol out of 12+8 = 20 ml total solution.
The alcohol concentration of the mix of A and B is 12.8/20 = 0.64 = 64%