sorry folks, I posted my question on wrong thread. It is in fact not a math question rather is IQ question. The answer goes like this... First switch on the first switch for 1 minute or so. then switch off the first switch and switch on the second one. Go upstairs... the bulb which is on is the one operated by switch 2, touch either of the off bulbs, the one which is warm is the one operated by the first switch and the cold one is the one operated by the third switch..