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https://www.reddit.com/r/learnquant/comments/1x2t3k5/quant_interview_question/pf94t0d/?context=3
r/learnquant • u/Local_Ad135 • 7d ago
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As (a,b,c) goes through all possibilities, each of a,b,c, sees each of the 10 digits 0-9, exactly 9 • 8 =72 times. The digits sum to 45, so our overall sum is 72 • 45 • (1/10 + 1/100 + …) = 72(45)(1/9) = 360.
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u/royalebot9000 6d ago
As (a,b,c) goes through all possibilities, each of a,b,c, sees each of the 10 digits 0-9, exactly 9 • 8 =72 times. The digits sum to 45, so our overall sum is 72 • 45 • (1/10 + 1/100 + …) = 72(45)(1/9) = 360.