7. (10 points) Uncle Cook's hat fell in front of the gate, still smoking. It turned out that someone had thrown a firecracker from the window, and the falling firecracker had set the hat on fire. When the accident happened, 5 boys all poked their heads out, and of course, none of them were willing to admit it. Now, 4 of the boys are telling the truth, and one person is lying, and the liar is the one who threw the firecracker. So, the liar's room number is $\qquad$Buster: “It wasn't me, I only found out what happened when Uncle Cook shouted.” Oak: “It wasn't me, Marco can vouch for me, I didn't throw anything.” Marco: “It wasn't Oak, it didn't come from above, I didn't see anything, and I didn't throw anything.” Corobi: “But I saw it, someone threw something from above.”

Marv: “Yes, someone threw something from above, it flew over my head, just grazing my hair.”
【Answer】Solution: Because:

Oak: “It wasn't me...”
Marco says, “It wasn't Oak...”, both statements are consistent, so both are telling the truth; Marco says, “…it wasn't thrown from above...”, Marco is in 401, so 502's Buster is excluded; the remaining people are Conobi and Malf,

Conobi says, “...someone threw something from above...”, but the previous analysis has already excluded 502, 402, and 401, so Conobi is lying,

Malf's statement confirms that Conobi is indeed lying.
Therefore, Conobi is the liar. His room number is 302.
So the answer is: 302.