dap an bay gio se duoc bat mi :phep cong da cho ta tong so cua , moi so co 3 chu so va ket qua la so co 4 chu so nen so * o tong phai la chu so 1 .
hai so co 3 chu so cong lai duoc 1997 thi 2 so do pai la 999 va 998
ta co phep tinh 999
+ 998
_______
1997
ai k mk mk k lai