Complex sentence: Periods 1,3,5 and 7 all last 35 minutes whilst periods 2,4,6 and 8 last 40 minutes, therefore all double periods have a duration of 75 minutes (1¼ hours).
Split into simpler sentences: Periods 1,3,5 and 7 all last 35 minutes whilst periods 2,4,6 and 8 last 40 minutes. Therefore all double periods have a duration of 75 minutes (1¼ hours).