Evaporation will not evaporate the salt, so the salt content in the obtained brine with a concentration of 3.5% is still 17.5g,

So the quality of this brine is 17.5 divided by 3.5% = 500g, so 700-500g of water is evaporated.

2.20% salt solution 1200g contains 20% * 1200 = 240g salt.

After adding 800g of water, the mass of salt is still 240g, and the total mass of brine is1200+800 = 2000g.

So the concentration is 240 divided by 2000= 12%.

In the future, you should pay attention to the invariants in these processes, such as evaporation and adding water, which will not change the quality of the solvent in the solution.