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Find the number of positive integers that satisfy both the following conditions:

Each digit is a 1 or a 2

The sum of the digits is 4

 Mar 27, 2023
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1 digit numbers: 0 {}

2 digit numbers: 1 {22}

3 digit numbers: 3 {112,121,211}

4 digit numbers: 1 {1111}

 Mar 27, 2023

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