103. a) Prove that the sum of all n-digit numbers $(n>2)$ is $494 \underbrace{99 \ldots 9}_{(n-3) \text { times }} 5 \underbrace{00 \ldots 0}_{(n-2) \text { times }}$ (thus, the sum of all three-digit numbers is 494550, and the sum of all six-digit numbers is 494999550000).

b) Find the sum of all four-digit even numbers that can be written using the digits $0,1,2,3,4,5$ (the same digit can be repeated in a number).