PERCENTILE_CONT

/* This function will returns the continuous interpolated value at the specific offset

PERCENTILE_CONT() function will takes a parameter, which indicates the offset and it should be between 0 and 1. */

DECLARE @T TABLE
(
GroupID INT,
Number INT
)

INSERT INTO @T (GroupID,Number)
VALUES (1,10),(1,20),(1,30),(1,50),(1,60)

SELECT GroupID,Number,
PERCENTILE_CONT(0.4) WITHIN GROUP (ORDER BY Number)
OVER (PARTITION BY GroupID) AS [InterpolatedAt0.4],
PERCENTILE_CONT(0.8) WITHIN GROUP (ORDER BY Number)
OVER (PARTITION BY GroupID) AS [InterpolatedAt0.8]
FROM @T