Question 967697
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<pre>
The total number of placing 4 persons in a row of 8 chairs is  {{{4!*C[8]^4}}} = 8*7*6*5 = 1680.

    Here {{{C[8]^4}}} represents the number of chosing chairs and 4! represents the number of all possible permutations 
    of 4 person inside the given claster of 4 chairs.


Now, there are 2 major configurations of favorable placings, a) and b)


  a)                            b) 
                     A  B                           B  A
   1  2  3  4  5  6  7  8         1  2  3  4  5  6  7  8


where April and Bobby occupy chairs ## 7 and 8.


For configuration a), we have  {{{2*C[6]^2}}} = 6*7 = 42 possible placings of two other persons.

For configuration b), we have  {{{2*C[6]^2}}} = 6*7 = 42 possible placings two other persons  (the same number as in a).


So the number of favorable placings is  42+42 = 84.


Now the probability under the question is  P = {{{84/1680}}} = 0.05.    <U>ANSWER</U>
</pre>

Solved.