program marks;
var
   mark:array[1..5] of integer;
   x:integer;
   counter:integer;
Begin
     counter:=1;
     x:=1;
     while (counter<=5) do
          begin
               writeln('input marks');
               readln(mark[x]);
               x:=x+1;
               counter:=counter+1;
          end;
 
     writeln(mark[3]);
 
end.

program array_sum;
Uses wincrt;
var
   nums:array[1..10] of integer;
   index:integer;
   Sum:integer;
Begin
     index:=1;
     writeln('Input marks');
     while (index<=10) do
           Begin
                Readln(nums[index]);
                index:=index+1;
           end;
 
     index:=1;
     while (index<=10) do
           Begin
                sum:=(nums[index]);
                index:=index+1
           end;
     writeln('sum',sum);
end.

sum:=(nums[index]);

program array_sum;
Uses wincrt;
var
   nums:array[1..10] of integer;
   index:integer;
   Sum:integer;
Begin
     sum:=0;//Delphi initializes local variables. Not sure if your Pascal compiler does
     index:=1;
     writeln('Input marks');
     while (index<=10) do
           Begin
                Readln(nums[index]);
                sum:=sum + nums[index];
                //you can update sum in the input loop - no need for a separate loop
                index:=index+1;
           end;
     writeln('sum',sum);
end.