With 2 elements pairs which give sum as 9 $= \{(1,8),(2,7),(3,6),(4,5)\}$ So choosing 1 elements from each group = 4 elements (in worst case 4 elements will be $\{1,2,3,4\}$ or $\{ 8,7,6,5\}$) Now using pigeonhole principle = we need to choose 1 more element so that sum will definitely be 9 so Number of elements = 4 +1 = 5
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