You guys are thinking too much for this question. If we know 75% of A is something, and 80% of B is something. Then if we combine the two we know the the total percentage would be between 75%-80%. And the final percentage is strictly dependent on the ratio of the two. If we get final percentage is 75% we know we have 100% A and if the final percentage is 80% we know we have 100% B. Otherwise it'll be somewhere in between.
Sometimes it's enough to just use your intuition.
Keep on asking, and it will be given you;
keep on seeking, and you will find;
keep on knocking, and it will be opened to you.