The first digit could be any of these: 1, 2, 3, 4, 5, 6, 7, 8, 9
There are 9 choices, and only one is 1, so, the probability is 1/9
The second digit could be these(because the number is even): 2, 4, 6, 8
There are 4 choices, and only one is 8, so the probability is 1/4
Multiply the fractions:
1/9*1/4=1/36
The probability of his house number being 18 is 1/36
Good luck!