cac chu so khac nhau duoc lap tu cac chu so 0,1,2,3,4,5,6,7,8,9
chu so hang nghin duoc chon 9 lan
chu so hang tram duoc chon 10 lan
chu so hang chuc duoc chon 8 lan
chu so hang don vi duoc chon 7
vay co tat ca cac so co 4 chu so khac nhau la:9x10x8x7=5040(so)