It would be pretty awful if "one out of every 5.2 couples divorced in 1990." It should read "there were 5.2 marriages for each divorce" [like the English translation].
「平均每五．二對夫妻有一對離婚」，這種說法太可怕了，應該是「每五．二次結婚有一次離婚」才合理。