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                        D U P L I C A T E 
I just responded once.
Below is the copy of that response.
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(a)   remains (3-i) inches, if i < 3;  
      if i = 3, remains 0 (same as nothing);
      if i > 3   or i < 0, then the problem makes no sense.
(b)  remains 3 - (i-5) = 8-i  inches, if  5 < i < 8;
     remains 3 inches, if i = 5;
     remains 0 inches, if i = 8;
     if  i < 5  OR  i > 8,  then the problem makes no sense.
Solved, with giving you full answers.