1. Find the sum of all three-digit numbers that can be formed from the digits $1,2,3$ and 4.
Solution. From the digits 1, 2, 3, and 4, $4^{3}=64$ three-digit numbers can be formed. Each digit appears in the last place the same number of times, i.e., each of the digits 1, 2, 3, 4 as the last digit appears in 16 three-digit numbers. Therefore, the sum of the units is:

$$
16(1+2+3+4)=160
$$

and the sum of the tens and hundreds will be 1600 and 16000, respectively. Therefore, the sum of all three-digit numbers will be

$$
160+1600+16000=17760
$$