A party of 23 persons take their seats at a round table. The odds against two persons sitting together are

10 : 1 1 : 11 9 : 10 None of these

10 : 1

$P=\frac{(21)!2!}{(22)!}=\frac{1}{11}=\frac{1}{1+10}$  ∴ odd against = 10 : 1