Maths percentage, Please teach me how to do, thanks?

Assume that there is y cm3 solution from B bottle to be mixed with 1000 cm3 solution from A bottle. Thus total alcohol in the mixture will be (1000)(30%)+(y)(90%)=300+9y and the total mixture has volume y+1000 cm3. Hence the minimum of y to make the resulting mixture has concentration 50% or more is the y such that the equation 300+9y=(50%)(y+1000) is satisfied. solving this equation yields y=500 cm3. Or equivalently, the resulting mixture has total volume 1500 cm3.

Thanks i knew how to do.