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let S be the set of natural numbers whose digits are chosen from {1,3,5,7} such that no digits are repeated find

1) |S|

2) $\sum (n)$ where n belongs to s

first part i know 64 numbers would be there how to approach 2nd part

1 Answer

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length 1: 1 or 3 or 5 or 7 $\sum \left ( 1+3+5+7 \right )$=16

length 2: 13,15,17,31,35,37,51,53,57,71,73,75=$\sum \left (13,15,17,31,35,37,51,53,57,71,73,75 \right )$=527

similarly

length 3: 10656

length 4: 106656

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