bội của 4 = 0,4 ,8. 12,16,20 ,24, ...... boi cua 5 = 0,5,10,15,20,25,30 ,........... boi cua 6 =0 ,6,12,18,24,30 ,..... boi cua 7 = 0,7,14,21,28........ boi cua 8 = 0,8,16,24 ...... boi cua 9 =0,9,27,36,.........
Boi cua 1 = 0 ,1 ,2 .3.4 ,5 .6 .7 .8 .9 .10 ........ Boi cua 2 = 0 .2 .4 .6 .8 .9 ........ Boi cua 3 = 0 . 3 . 6 . 9 ..................