5 ft. First, let's see how long of a piece of glass Mark needs in nice simple terms. sqrt(20) ft = 4.472 ft Now let's see which of the 4 pieces of glass he has is at least as long as 4.472 ft, and is the shortest of those that are at least that long. 6ft. Long enough. Let's see if there's something shorter. 5ft. Also long enough and a better choice than the 6 foot piece. 3ft. Too short. Best choice so far is still 5 feet. 7ft. Long enough, but longer than the best so far of 5 feet. So the best choice is to cut the piece of glass that Mark needs from the 5 foot long piece.