the answer is that each side is 6x feet long
Answer:
First do the case in which none of the digits is 0.
There are 9 ways to choose the digit that repeats 3 times and 8 ways to pick the other, after this there are 4 ways to order the digits. So 9×8×4.
Now we look at the cases in which the digit 0 repeats 3 times. Clearly the 9 numbers are 1000,2000,3000,…,9000.
Lastly we look at the cases in which 0 appears once. There are 9 ways to pick the other digit and 3 ways to order.
So the final answer is 9×8×4+9+9×3.
