Light one of the threads from 1 end, while light the other thread from both ends. The second thread will burn completely after 1/2 hr or 30 minutes. At this time, the first thread has also burnt for 30mins. Now light it from both ends. It would also begin to burn at double speed, and will last for another 30/2 or 15 minutes.
So total time measured will be 30 + 15 or 45 minutes.
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