Outside a room are 3 switches, each controlling one of 3 old-style bulbs inside the room (which…

Why

Divya's reasoning assumes there are only two things to check once inside — is each bulb ON or OFF — and with only 2 states for 3 bulbs, one visit genuinely could not tell them apart. The trick is that an old-style bulb leaves a THIRD clue behind: heat. Turn switch 1 on and leave it a couple of minutes, then turn it off and turn switch 2 on, leaving switch 3 off the whole time — now walk in once. The bulb that is glowing is switch 2's; the bulb that is off but still warm to the touch is switch 1's (it was on recently); the bulb that is off and cold is switch 3's (it was never turned on). Test it: this only works with old-style bulbs that actually heat up when lit — most LED bulbs stay cool even after minutes on, which is exactly why the puzzle specifies an old-style bulb. Her friend is right: using heat as a third clue, alongside on/off, one visit is enough to match every switch to its bulb.