Answer:
yes another flavor should be in the bag. because the problem stated that there are three different flavor, and only two flavors were mentioned in the problem. the probability of getting the third flavor is 5/7 i guess, because it think the probability of the second flavor was the answer when you added both the numerator and the denominator of 1/3 by 2. like 1/3+2=3/5, so if you added 2 to both the numerator and the denominator of 3/5, you will get 5/7. but i am not so sure about this, i'm sorry in advance.
3 is the common difference
