If 20Cr + 1 = 20Cr − 1 , then r is equal to
(a) 10
(b) 11
(c) 19
(d) 12
$r+1+r-1=20 \quad\left[\because{ }^{n} C_{x}={ }^{n} C_{y} \Rightarrow n=x+y\right.$ or $\left.x=y\right]$
$\Rightarrow 2 r=20$
$\Rightarrow r=10$
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