So with 200 grams of liquid, you have 40% juice and 60% water.
200 * 40% = 200 * .4 = 80 grams of juice
200 * 60% = 200 * .6 = 120 grams of water
Now we add 200 more grams of water.
120 + 200 = 320 grams of water
More importantly...
200 + 200 = 400 grams of liquid
Now we need the percent of juice in the new mix. Note that now there is 400 grams of liquid, as we started with 200 grams and added another 200 grams.
80 / 400 = 0.2 = 20%
For a quick reality check, you can see this answer makes sense. You are doubling the amount of overall liquid, but the amount of juice stays the same. The % juice is halved from 40% to 20%, so the answer makes sense.
