Answer:
36 or 6 (read the explanation, it'll all come together)
Explanation:
First, you enter a room already filled with 34 people, so the total number of people in the room is 35.
Then, the shooter enters, making the total 36, and then kills 30 of them.
I think we can all agree on these two statements.
However, how many people are left in the end are actually depending on how the individual thinks about it.
See what I mean: Some may think that when the shooter kills 30 of the people in said room, then there are six remaining, right?
I mean, they are dead, so they don't belong in the room... or do they?
Some may think that when the 30 die, they are officially out of the room (perhaps in heaven or whatever afterlife you believe in??), and if you think this, then the answer is 6 people.
However, If you say that the 30 people who died is still in the room (as one can make the argument that their bodies are still physically in the room), then the total is still 36.
Hope this helps :)