The Two Strings Puzzle

No matter how uneven the burn rate is, if you light both ends of either string at the same time, that string will burn up in 30 seconds. Therefore, arrange the strings so that you can light one end of string A and both ends of string B all at the same time. As soon as string B burns up (in 30 seconds), snuff out the burning string A, which will leave it still having 30 seconds left to burn. Now, by lighting both ends of string A at the same time, you will have a 15-second timer, for that is how long string A will burn.