This is to grasp that the salt contained in it has the same final weight, and the rest is to solve the equation, and there is nothing else.

So let's assume that x grams of salt water with 8% salt content is needed, and the equation is:

200*5%+x*8%=700*6%

The solution is x=400.

So 700-400-200= 100.

Therefore, it is necessary to add100g water.