so dan o 1 xa hien nay co 4000 nguoi.
a/ Voi muc tang hang nam la cu 1000 nguoi thi tang them 21 nguoi, hay tinh xem 1 nam sau so dan cua xa do tang them bao nhieu nguoi?
b/ Neu ha muc tang hang nam xuong la cu 1000 nguoi chi tang them 15 nguoi, thi sau 1 nam so dan cua xa do tang them bao nhieu nguoi?