Demetri is a participant in an auditory detection study using the method of constant stimuli. He never detects the 10 unit tone. He detects the 20 unit tone 25% of the trials. He detects the 30 unit tone 50% of the trials. He detects the 40 unit tone 80% of the trials. He detects the 50 unit tone 95% of the trials. His threshold for hearing tones would be taken as the _________.