Hi!
The first thing I would do here is order the set of numbers, from smallest to largest. The order would be:
1, 2, 3, 3, 4, 5, 7, 9, 9, 9, 12, 13, 14, 15, 15, 16, 17, 17, 18, 20
The percentiles would be in the set, the number at the position of 30% and 90% out of the values in the entire set.
So essentially, since there are 20 values, you multiply 20 with 30% and 90% with 20 values, and you'll get the number of the value in the set which is at that percentile.
30% * 20 = 6
90% * 20 = 18
Now, just find the 6th and the 18th value.
The 6th is 5, and the 18th 17.
So your answer is, 30th percentile ~ 5, 90th percentile ~ 17
Hope this helps!
