Table:
0
A
B
C
1
council tax id
cmi cross reference id
rent arrears
2
1
100
3
2
8
4
6
59
5
7
2

Questions and formulas:
Question: How many council taxes are collected for renting arrears ?
Formula: SUMMARIZE(C1,SUMX(A1))
Formula2: SUMMARIZE(C2:C5,SUMX(A2:A5))
Level: easy
Question: Number of council taxes and renting arrears.
Formula: HSTACK(SUM(A1), ROWS(UNIQUE(C1)))
Formula2: HSTACK(SUM(A2:A5), ROWS(UNIQUE(C2:C5)))
Level: hard