Question 1183064
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            In my response,  I will answer part  (a),  ONLY,  because it is really good entertainment problem/question, 

            while the part  (b)  is rather technical   -   so,  the two parts differ by their style.



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The answer is  {{{5!/2}}} = {{{(1*2*3*4*5)/2}}} = {{{120/2}}} = 60 strings,


because exactly half of all 120 permutations have A appeared before D, 
while the other half of the 120 permutations have A appeared after D.
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Part (a) is answered.