The 90th percentile of the data is the value which is greater than 90% of the data. Arranging the values first in ascending order:
1, 2, 5, 5, 7, 7, 9, 11, 11, 12, 12, 13, 15, 16, 17, 17, 18, 19, 20, 20
The data has 20 elements, so 90% of the data consists of .90(20) = 18. The 90th percentile must be the 19th element. Therefore, 20.
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