From 100 - 199, These are the numbers containing a zero:
100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 120, 130, 140, 150, 160, 170, 180, 190
There are 19 of them.
From 100 - 199, there are 19 .
From 200 - 299, there are 19 .
From 300 - 399, there are 19 . etc.
From 100 - 999, there are 19 * 9 numbers containing zero.
And there are 900 numbers from 100 - 999, including 100 and 999.
So the probability = \(\frac{19*9}{900}\,=\,\frac{171}{900}\) ???