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You have 2 safety fuses and 4 matches. Each fuse takes exactly one hour to burn, but they are not perfect so may not necessarily burn evenly from both sides. For example, the first half might burn in 20 minutes and the second half might take 40 mins. but this does not change the total time to be burnt as stated.
How can you measure out 45 minutes by just using these two safety fuses and the matches you have?
Answer:
Light the first fuse at both ends and the second fuse at one end. Once the first fuse is completely burned, light the other end of the second fuse. Once the second fuse is completely burned, 45 minutes will have passed.
Explanation:
With the first fuse burning at both ends, it will take 30 minutes to burn completely. At this point, the second fuse will have 30 minutes remaining when burning at one end; lighting the other end will burn the rest of the second fuse in $30/2 = 15$ minutes. The total time taken is $30 + 15 = 45$ minutes.
If the two fuses are labeled A and B:
Light both ends of fuse A, and one end of fuse B. Fuse A will completely burn out after 30 minutes. By this time, there are still 30 minutes of burn time in fuse B. Light the other end of fuse B when A finishes burning, which will halve the remaining 30 minutes of fuse B to 15 minutes, which when added to fuse A's 30 minutes will total 45 minutes.
Two matches are enough hehe... Make a circle from one fuse and light at the touching point. When ut is burnt it will be 30 mins. You prepaired the second one by cutting half, making two circles and placing them down with the four free ends touchin each other. You light it after the first rope is burnt down. It will take additional 15 minutes
