WITH boobies AS (SELECT Rate, ntile(20) OVER(ORDER BY Rate) AS n, COUNT(*) OVER(ORDER BY rate) AS x FROM RateCalculatorLookups)
--select * from boobies
/*
rate, n, x
NULL    1   3
NULL    1   3
NULL    2   3
10  2   4
14  3   5
15  3   6
20  4   7
37  4   8
42  5   9
44  5   10
45  6   11
50  6   16
50  7   16
50  7   16
50  8   16
50  8   16
51  9   18
51  9   18
55  10  20
55  10  20
56  11  21
58  11  22
59  12  23
65  13  25
65  14  25
73  15  26
85  16  27
87  17  28
91  18  29
100 19  30
900 20  31*/
SELECT avg(Rate) FROM boobies WHERE (x >= 20 AND n BETWEEN 2 AND 19) OR (x < 20 AND n BETWEEN 2 AND x-1)
-- 52,8888888888889