Let's look at the initial and the final weeks of the rankings.  During this time, the song initially in the top position can drop at most 19 spots to position 20; the second ranked song can drop at most 18 spots to position 19, and so on. This can take at most 19 + 18 + ... +  2 + 1 = 190 weeks.  Therefore there are at most 191 weeks in which the same songs can appear on the charts.  You can achieve this maximum, by switching only two songs each week.  

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ã2004 Alberto L. Delgado