Table:
0
A
B
C
D
1
student id
course id
registration date
student course registrations
2
111
301
2008-11-04 10:35:13
3
121
301
2008-10-04 10:35:13
4
121
303
2008-11-14 10:35:13
5
131
303
2008-11-05 10:35:13
6
141
302
2008-11-06 10:35:13
7
151
305
2008-11-07 10:35:13
8
161
302
2008-11-07 10:35:13
9
171
301
2008-11-07 10:35:13
10
141
301
2008-11-08 10:35:13

Questions and formulas:
Question: What are the students who registered for some courses that had the least number?
Formula: TAKE(SORT(SUMMARIZE(A1,SUMX(B1)), 2, 1), 1)
Formula2: TAKE(SORT(SUMMARIZE(A2:A10,SUMX(B2:B10)), 2, 1), 1)
Level: hard