[Originally posted in Discussions]
Yes, you could! The reason Dr. Loh doesn't focus on this way is because you might have thought of that already, and he wants to show you a neat, different way of doing it.
The casework method would be as follows:
Case 1: All different digits
The digits have to be 1, 2, and 3, and there are 3! = 6 ways to mix them up to create a three-digit number.
Case 2: Two same digits, the third different
There are three possibilities for the double-digit: 1, 2 or 3. After we have chosen the double-digit, we have two choices for the other, different digit. This gives 3×2 = 6 ways. However, we can mix them up. For example, 122 can be mixed up in three ways (122, 212, and 221). So we have 6×3 = 18 ways for this case.
Total: Case 1 (6) + Case 2 (18) = 24 total ways.
There's an urge in math-aficionados and math-philes to always search for new, clever or surprising ways of solving problems. It's the same urge that makes a computer programmer spend hours writing a program to do a mundane calculation that might take him the same amount of time if he did it by hand. And it's the same urge that makes you want to tell a little kid to count his legos by making piles of 10, rather than counting one by one.
If you still have any more questions, please don't hesitate to ask!
Happy Learning,
The Daily Challenge Team