Aaron is training for a 10km marathon swim in 18 weeks. Each week, his trading plan includes a long distance swim. Over the first 6 weeks, Aaron swam a total of 18.75km. Over the first 10 weeks, Aaron swam a total of 36.25km. Each week, Aaron increased the distance he swam following an arithmetic sequence until the final week. a. By what distance did Aaron increase his total swimming each week?