This was a question asked in a Personal Interview of Microsoft corporation..
You have two ropes exactly of same length.These two burn with a uniform constant rate(not necessary that you burn them simultaneously) and are burned up completely in 30 minutes.Also you have been provided with matchsticks.Using these materials how will you measure 45 minutes???
Details You can't fold the rope and you do NOT have a measuring instrument. Inaccuracy may be allowed if its not too large(because you have to imagine it practically). There is sufficient number of matchsticks