A number written as the sum of the values of it's digits?

Answer 1

If it's a one-digit number then any 1-digit number will do (0...9)

2-digits numbers : ab=10a+b=a+b it's not possible unless a=0

For 3-digits: abc=100a+10b+c=a+b+c => 100a+10b = a+b it' not possible unless a=b=0

And so on...