Question

How many 4-digit numbers can you write if you cannot use any other digits except 0, 1, and 2?

Answer 1

You have a choice of only 3 digits to fill in 4 places.
You can place any of 3 digits in each of the 4 places, so you get
3 * 3 * 3 * 3 = 81

Answer 2

Think of the four digit number like a slot. Each slot has 3 different possibilities so that makes the total slot have 3*3*3*3 probabilities, so 81