As part of a Mission Impossible team, Alex has a vital switch to throw in exactly 31 minutes. Unfortunately, his watch has just stopped. All he has are two lengths of fuses, which burn irregularly, and a supply of matches.

One fuse takes 50 minutes to burn completely when lit from either end, and the other which burns in 24 minutes.

How does Alex use these two fuses to time exactly 31 minutes? (A fuse can not be folded to make a shorter time, but it can be burned from both ends to get half the time.)