Form 20. It is a question of permutation and combination, and this number only consists of 0 and 9. First of all, the composition of a digit is 9. Secondly, the two digits are 90 and 99. These three numbers are 900, 909 and 990. These four numbers are 9000, 9009, 9090 and 9900. These five numbers are 90000, 90009, 90090, 90900 and 99000. These six figures are 900000, 900009, 900090, 900900, 909000 and 990000.