Probability, at least case, minimum length, random no

These are not homework. I can not continue. please help
A software function generates a random number with a length of N digits. Each digit is determined by random selection of a value between 0 and 9. All ten values ​​are equally probable, and different numbers can have the same value. Determine the minimum length N so that with a probability of at least 50%, at least one digit has the value 0.