12. Among all two-digit numbers, how many two-digit numbers are there where the tens digit is greater than the units digit?
12. There are 45 such two-digit numbers
12.【Solution】Among the two-digit numbers that meet the requirement, the unit digit is less than the tens digit. They can be listed as follows: tens digit unit digit
10
20, 1
$30,1,2$
$90,1,2, \cdots, 8$
Therefore, the total number of two-digit numbers that meet the requirement is
$$
1+2+3+\cdots+9=\frac{(1+9) \times 9}{2}=45(\text { numbers) }
$$