Solution

Let's mark the switches as "A", "B" and "C".

Press the switch "A" to the "on" position and keep it in this position for several minutes (say five minutes). After that put it in the "off" position. Immediately put switch "B" in the "on" position and, now, rush to the roof top.

Touch each of the bulbs. The bulb which is not lighted and is cold to the touch, matches switch "C". The bulb which is not lighted but is warm to the touch matches switch "A" and the bulb which is lighted matches switch "B".

Some useful links for
your career:


  • Union Public Service Commission - www.upsc.gov.in
  • IIT-Kharagpur - www.iitkgp.ac.in
  • Indian Statistical Institute - www.isical.ac.in
  • Indian Institute of Technology Madras - www.iitm.ac.in
  • Indian Institute of Management, Ahmedabad - www.iimahd.ernet.in
  • Indian Institute of Mass Commission - www.iimc.nic.in
  • IIT Bombay - www.iitb.ac.in
  • Indian School of Mines, Dhanbad - www.ismdhanbad.ac.in
  • Birla Institute of Technology, Ranchi - www.bitmesra.ac.in
  • Central Institute of Fisheries Nautical and Engineering Training - www.cifnet.nic.in
  • Indian Institute of Information Technology, Allahabad (Deemed University) - www.iiita.ac.in
  • Central Marine Fisheries Research Institute, Kochi - www.cmfri.com
  • Tata Institute of Social Sciences, Mumbai - www.tiss.edu