Try this:

Program Averages;
Var
Counter: Integer;
Marks: real;
Infile: Text;
Total: Real;
Average: Real;

Begin
Counter:= 0;
Assign(Infile, 'Marks.txt');
Reset(Infile);
while not Eof(InFile) do
begin
ReadLn(InFile, Marks);
Counter := Counter + 1;
Total := Total + Marks;
end;
Average := Total/Counter;
WriteLn('The average is: ' ,Average:2:2);
Close (Infile);
End.

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