I thought of this a different way. I sliced each rope (length ways and not easy) into 4 equal narrow strips. Ending up with 8 strips. Tie any of the 3 strips together to make one long strip – light one end at it should burn for 45 minutes. I’d say this is more elegant since you have rope left over. Especially if you are on a desert island….

The better mathematical explanation is that using the base12 numbering system 12 in bas12 is the same as 14 in the base10 numbering system. Therefore 7 in base12 is half of 12 in base12………