WITH totals AS
 (
 SELECT score_description,COUNT(*) AS student_count, score,
    ( SELECT COUNT(*) FROM subtest_registration )  AS total_students  -- WHERE subtest_id = 'e696b230-10e0-11df-80f4-1a7fbecf21f7'
   FROM subtest_registration_result
   INNER JOIN subtest_registration ON subtest_registration_id=id
   WHERE score_type_id = 2
   GROUP BY score_description, score
 )
SELECT score_description, student_count,
total_students,
( student_count * 100.00 ) /  total_students as percentage
 FROM totals;


 score_description | student_count | total_students |     percentage
-------------------+---------------+----------------+---------------------
 M                 |          7803 |          40321 | 19.3521986061853625
 B                 |          7996 |          40321 | 19.8308573696088887
 E                 |         14596 |          40321 | 36.1994990203615982
 W                 |          9926 |          40321 | 24.6174450038441507
(4 rows)


real    0m0.078s