An airline surveyed business travelers to evaluate their flying habits. Out of 325 persons surveyed, 56 of the respondents fly once a year; 93 of the respondents fly three times a year; 67 of the respondents fly four times a year; 81 of the respondents fly five times a year; and 28 of the respondents fly seven times a year. Determine the mode of the number of times per year that respondents fly.