Table:
0
A
B
C
D
E
1
Year
1851
1861
1871
1881
2
Year
1851
1861
1871
1881
3
Population
1027
992
1017
813
4
Year
1961
1971
1981
1991
5
Population
634
683
687
764
6
Census: 1801– 2001 2011
Census: 1801– 2001 2011
Census: 1801– 2001 2011
Census: 1801– 2001 2011
Census: 1801– 2001 2011

Questions and formulas:
Question: What's the 1851 when 1881 had 764?
Formula: UNIQUE(CHOOSECOLS(FILTER(A1:E1,E1=764),2))
Formula2: UNIQUE(CHOOSECOLS(FILTER(A2:E6,E2:E6=764),2))
Level: medium
Question: What's the year when 1851 had 634?
Formula: UNIQUE(CHOOSECOLS(FILTER(A1:E1,B1=634),1))
Formula2: UNIQUE(CHOOSECOLS(FILTER(A2:E6,B2:B6=634),1))
Level: medium
Question: What's the 1881 when 1871 had 1017?
Formula: UNIQUE(CHOOSECOLS(FILTER(A1:E1,D1=1017),5))
Formula2: UNIQUE(CHOOSECOLS(FILTER(A2:E6,D2:D6=1017),5))
Level: medium
Question: What's the 1861 had 1991 in 1881 and a year of year?
Formula: UNIQUE(CHOOSECOLS(FILTER(A1:E1,(A1="year")*(E1=1991)),3))
Formula2: UNIQUE(CHOOSECOLS(FILTER(A2:E6,(A2:A6="year")*(E2:E6=1991)),3))
Level: medium
Question: What's the 1881 with 1961 in 1851?
Formula: UNIQUE(CHOOSECOLS(FILTER(A1:E1,B1=1961),5))
Formula2: UNIQUE(CHOOSECOLS(FILTER(A2:E6,B2:B6=1961),5))
Level: medium