B3:

program bt;
uses crt;
var i,n,s,d,a:integer;
begin
 clrscr; i:=1;
 write('Nhap a: '); readln(a);
 write('Nhap d: '); readln(d);
 write('Nhap n: '); readln(n);
 while (i<=n) do
 begin
   s:=s+a+(i-1)*d;
   inc(i);
 end;

 writeln('Tong: ',s);
 readln;
end.

B4:

program bt;
uses crt;
var i,n,s,q,a,t,j:integer;
    d:real;
begin
 clrscr;  i:=1;
 write('Nhap a: '); readln(a);
 write('Nhap q: '); readln(q);
 write('Nhap n: '); readln(n);
 // for i:=1 to n do t:=q*t;
 //  d:=a*(1-t);
 //  writeln('Tong: ',d/(1-q):0:3); {Neu xai cong thuc lop 11 thi xai cai nay bo het cai duoi ko thi thoi}
 while (i<=n) do
  begin
    t:=1;
    for j:=1 to i-1 do t:=q*t;
    s:=s+a*t;
    inc(i);
  end;
 writeln('Tong: ',s);

 readln;
end.

B6:

program bt;
uses crt;
var b,c:integer;
begin
 clrscr;
 write('Nhap tuoi ba: '); readln(b);
 write('Nhap tuoi con: '); readln(c);
 if (b-c<=19)or(c>b)or(b<0)or(c<0) then write('Ko hop le')
 else if (b/c=2) then write('Tuoi ba gap doi duoi con')
 else write('Tuoi ba ko gap doi tuoi con');
 readln;
end.

Bài đồng hồ mik đang suy nghĩ, khá là khó