1:

uses crt;
var d:text; tong,i:longint; a:array[1..100000]of longint;
begin
clrscr;
   assign(d,'data.inp');reset(D);
      while not(eof(d)) do begin inc(i); readln(d,a[i]); inc(tong,a[i]) end;
   close(d);
       writeln(tong/i:0:2);
readln;
end.

2:

uses crt;
var d:text; i,n:longint; a:array[1..100000]of longint;
begin
clrscr;
   assign(d,'input.pas');reset(D);
      while not(eof(d)) do begin inc(n); readln(d,a[n]); end;
   close(d);
   assign(d,'output.pas');rewrite(D);
      for i:=1 to n do
         if a[i] mod 3=0 then writeln(d,a[i]);
   close(d);
end.
3:

uses crt;
var a,b,c:longint; tong:real;
function dt(a,b,c:longint):real;
begin
   dt:=((a+b)*c)/2;
end;
begin
clrscr;
   write('Do dai day tren, do dai day duoi, do cao cua thua ruong 1:');readln(a,b,c);
   tong:=tong+dt(a,b,c);
   write('Do dai day tren, do dai day duoi, do cao cua thua ruong 2:');readln(a,b,c);
   tong:=tong+dt(a,b,c);
   writeln(tong:0:2);
readln
end.

Câu 4:mình cũng ko biết làm nữa