The only way to get 80 books on the same science is either physics or chemistry, of the others there are not even 80 books in the room.
On the whole we have 365 books in the room.
We now look for the most unfavourable situation:
We could pick 50 biology books first.
Then we pick 65 zoology books
and then 50 geology books.
So now we removed 165 books from the room and still do not have 80 books of one kind.
This leaves us with 365-165=200 books in the room.
Now we have only physics and chemistry books left, both of which can sum up to 80 books.
It could now happen that we pick 79 physics books for example (the same applies to the chemistry books as well)
We then have picked 244 books out of those 365 books, which leaves us with 121 books.
Now, we are still looking at the most adverse situation!, we only pick chemistry books. We have to do this until we have picked 80 books.
So, 121 books left - 80 books = 41 books.
Therefore, of those original 365 books we have 41 left which we did not pick.
Thus, we had to pick 324 (365-41) books in order to make sure that we will definitively have 80 books that cover one special science field.
Hope I could help.
