What is the sum of all 3-digit numbers that can be constructed using the digits 3,4 and 5, if each digit can used only once in each number?

Answer :

345-354-435-453-534-543 six ways to arrange them which equals 2664.

This is another approach using the knowledge of place value.
100(24)+10(24)+(24)= 2400+240+24=2664.

My question is from where we got 24 ? What I knew is each number appears twice in hundreds, tens, and ones columns.