Table:
0
A
B
C
D
1
document id
mailed to address id
mailing date
documents mailed
2
2
8
1977-04-01 17:03:50
3
4
3
1992-11-07 15:03:41
4
4
9
1973-02-21 10:17:01
5
7
5
1979-09-21 10:30:52
6
10
3
1993-05-24 22:13:48
7
12
0
1999-05-22 23:21:07
8
12
7
2007-01-01 22:32:11
9
12
8
2007-03-20 05:22:01
10
13
4
1991-05-27 14:17:37
11
14
5
1986-05-16 06:25:33
12
20
2
2010-11-04 04:00:16
13
20
7
1982-01-14 05:50:54
14
23
8
1971-11-03 12:32:14
15
24
0
2013-01-27 03:29:31

Questions and formulas:
Question: when was document 7 mailed?
Formula: UNIQUE(CHOOSECOLS(FILTER(A1:D1,A1=7),3))
Formula2: UNIQUE(CHOOSECOLS(FILTER(A2:D15,A2:A15=7),3))
Level: medium
Question: the mailing date of No.7 document.
Formula: UNIQUE(CHOOSECOLS(FILTER(A1:D1,A1=7),3))
Formula2: UNIQUE(CHOOSECOLS(FILTER(A2:D15,A2:A15=7),3))
Level: medium