How many three digit numbers can be formed by using the digits 0, 1, 3, 5, 7 while each digit may be repeated any number of times ?

Total number of digits = 5
We cannot have 0 at the hundred's place so, the hundred's place can be digits with any of the 4 digits 1,3,5 or 7, So, there are 4 ways of filling the hundred's place can be digits with any of the 4 digits 1,3,5 or 7, So, there are 4 ways of filling the hundred's palce.
Since, the digit may be repeated in three digits numbers.
$\therefore$ Ten's place can be filled with any of the 5 digits in 5 ways
And unit's place can be filled with any of the 5 digits in 5 ways
Hence, the total number of required numbers
= $4\times 5\times 5=100$