Question 531044
<pre>
There are 9 ways to pick the 1st digit

For each of those 9 ways there are 8 ways to choose the second digit.
That's 9×8 or 72 ways to pick the first two digits.

For each of those 72 ways there are 7 ways to choose the third digit.
That's 72×7 ways or 504 ways to pick all three digits.

Edwin</pre>