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Combinatorics
hard
0:00.0
If
P
(
n
,
r
)
=
2520
P(n, r) = 2520
P
(
n
,
r
)
=
2520
and
C
(
n
,
r
)
=
21
C(n, r) = 21
C
(
n
,
r
)
=
21
, find
r
r
r
.
A
7
B
3
C
10
D
5
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