i think its pretty simple.... you can omit 555 from the problem, as it is redundant and at the start of each number, so you are left with four possible digits. surely you can start at the number 0000, and then 0001, and then 0002 etc... to reach the eventual 9999.

therefore you can realise that there are only 10000 possibilities. from 0000 to 9999 inclusive.