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Combinatorics
hard
0:00.0
What is the coefficient of
x
5
x^5
x
5
in
(
1
+
x
)
7
(1+x)^7
(
1
+
x
)
7
?
A
21
21
21
B
35
35
35
C
7
7
7
D
28
28
28
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