Maths question

Let x cm cube be the required amount of the liquid in bottle B should be used to mix.

Note that in the mixture, the total volume is (1000 + x) cm cube.

The amount of alcohol (in cm cube) in the mixture is
1000(30%) + x(90%)
= 300 + 0.9x

Therefore,
(300 + 0.9x) / (1000 + x) ≥ 50%
300 + 0.9x ≥ 500 + 0.5x
0.4x ≥ 200
x ≥ 500

Therefore, at least 500 cm cube the liquid in bottle B should be used to mix with it so that the mixed liquid formed contains at least 50% alcohol.