Mot don vi bo doi khi xep hang co 20 nguoi 25 nguoi hoac 30 nguoi deu thua 15 nguoi . neu xep moi hang 41 nguoithi vua du ( ko co hang nao thieu. Ko co ai o ngoai hang).hoi don vi co bao nhieu nguoi . biet rang so nguoi cua dom vi chua den 1000 nguoi