Let Bottle A have an indigo solution of strength 33% while Bottle B have an indigo solution of strength 17%. The ratio in which we mix these two solutions to obtain a resultant solution of strength 21% :
A
B
=
21−17
33−21
=
4
12
or
1
3
Hence, three parts of the solution from Bottle B is mixed with one part of the solution from Bottle A. For this process to happen, we need to displace 600 cc of solution from Bottle A and replace it with 600 cc of solution from Bottle B {since both bottles have 800 cc, three parts of this volume = 600cc}.As a result, 200 cc of the solution remains in Bottle B. Hence, the correct answer is 200 cc