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KEAM 2019 Math Paper
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© examsnet.com
Question : 32
Total: 121
If
n
C
r
−
1
=
10
,
n
C
r
=
45
and
n
C
r
+
1
=
120
, then
r
equals
1
2
3
5
4
Validate
Solution:
Using
n
C
r
n
C
r
−
1
=
n
−
r
+
1
r
n
C
r
n
C
r
−
1
=
45
10
and
n
C
r
+
1
n
C
r
=
120
45
⇒
n
−
r
+
1
r
=
9
2
and
n
−
r
r
+
1
=
8
3
⇒
8
3
(
r
+
1
)
+
1
=
9
2
r
⇒
16
r
+
16
+
6
=
27
r
r
=
2
© examsnet.com
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