On her vacations Veena visits four cities $( A ,\, B ,\, C$ and $D )$ in a random order. What is the probability that she visits $A$ just before $B$ ?
On her vacations Veena visits four cities $( A ,\, B ,\, C$ and $D )$ in a random order. What is the probability that she visits $A$ just before $B$ ?
$S=\left\{\begin{array}{l}A B C D, A B D C, A C B D, A C D B, A D B C, A D C B \\ B A C D, B A D C, B D A C, B D C A, B C A D, B C D A \\ C A B D, C A D B, C B D A, C B A D, C D A B, C D B A, \\ D A B C, D A C B, D B C A, D B A C, D C A B, D C B A\end{array}\right.$
Let I be the event "she visits A just before B"
$I =\{ ABCD , ABDC , CABD , CDAB , DABC , DCAB ,\}$
$So , n ( I )=6$
$P(I)=\frac{n(I)}{n(S)}$ $=\frac{6}{24}=\frac{1}{4}$
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