Question

How many 4-digit numbers are there, when a digit may be repeated any
number of times?

Answer 1

i think it will be 24

Answer 2

so you have to have a idea of permutations there are 10 digits from 0 to 9 so 0 cant be is first place so the 1st place can be arranged 10 -1 ways and the other three places in 10 ×10 ×10 ways
 the answer is  9× 10 ×10 ×10 = 9000 ways is the answer