



 
Hi Marcel, First of all, what you have already noticed is that if you have a collection of digits and the sum of the digits is 2 then either one of the digits is 2 an the rest are 0 or two of the digits are 1 and the rest are 0.
Let's jump to $n$ digit numbers, that is numbers from $10^n$ to $10^{n+1} 1.$
Does this help? Penny 



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