There were 300 participants at an art competition. 240 participants completed their drawings in time. What percentage of the participants did not complete their drawings?
To find the number of participants who did not complete their drawings, subtract the number of participants who completed their drawings from the total number of participants: 300 participants - 240 participants = <<300-240=60>>60 participants
Then divide the number of participants who did not complete their drawings by the total number of participants and multiply by 100% to find the percentage: 60 participants / 300 participants * 100% = 20%. Answer: \boxed{20}.