18. How many different two-digit numbers can be formed from the digits: a) $1,2,3,4,5,6$; b) $0,1,2,3,4,5,6$?
18. a) The choice of both the first and second digit can be made in six ways. In total, we will have $6 \cdot 6=36$ two-digit numbers.

b) The choice of the first digit can be made in six ways (zero cannot be taken), and the choice of the second digit can be made in seven ways. Thus, we will have $6 \cdot 7=42$ two-digit numbers.