Indigo Solution Replacement

Let $V$ cc be the volume taken from the second bottle and mixed into the first bottle (after throwing away $V$ cc from the first bottle).

Resulting concentration in the first bottle is $21\%$. By alligation / weighted average: $33\%(800 - V) + 17\%(V) = 21\%(800)$ $33(800 - V) + 17V = 21 \times 800$ $26400 - 33V + 17V = 16800$ $16V = 9600 \implies V = 600$

Since $600\text{ cc}$ was taken from the second bottle, the volume left in the second bottle is: $800 - 600 = 200\text{ cc}$.