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How many different whole numbers are there containing only the
digits 1 and/or 2 (each of these digits can be used one or more
times or not at all) such that for each of these numbers, the sum of all
of its digits equals seven?

 May 7, 2020
 #1
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If I understand your question, then there are 21 such numbers as far as I could tell as follows:

 

(1222, 2122, 2212, 2221, 11122, 11212, 11221, 12112, 12121, 12211, 21112, 21121, 21211, 22111, 111112, 111121, 111211, 112111, 121111, 211111, 1111111)>>Total = 21 such numbers

 May 7, 2020

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