Variant I heard, had a physicist, engineer, and mathematician sharing a room.
The first two wastepaper basket fires were taken care of the same ways by the physicist and engineer.
Then a third fire appears, but on the curtains. The mathematician says he'll take care of it, so the others go back to sleep. The mathematician removes the curtains, puts them in the wastepaper basket, and says, "Now it's a solved problem." He then goes back to sleep.
The first two wastepaper basket fires were taken care of the same ways by the physicist and engineer.
Then a third fire appears, but on the curtains. The mathematician says he'll take care of it, so the others go back to sleep. The mathematician removes the curtains, puts them in the wastepaper basket, and says, "Now it's a solved problem." He then goes back to sleep.