Question 617261
Because the 3-digit number must be less than 520, the digit in the hundreds place can only be 4.
Then you can pick any of the remaining 4 numbers for the tens place, leaving the other 3 numbers as possible choices for the units digit.
All in all, you get 4 choices for the tens place, and for each of those choices, 3 choices for the units place.
The total number of possible 3-digit numbers is 4 times 3, {{{highlight(12)}}}.