First of all, the total * * * has 90 double digits. One digit can be any integer between 0 and 9, and ten digits can be any integer between 1 and 9. According to the fact that the decimal digits are 9, 8, 7 ... 1, we can get a rule that there are 9+8+7+6+5+4+3+2+ 1=45 qualified numbers.