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Lucky seven III (Posted on 2011-01-05) Difficulty: 3 of 5
N is a base ten positive integer formed by writing the digit 7 precisely 2010 times, that is N = 77....77 (2010 times).

Determine the digital root of [N/199].

Note: [x] denotes the greatest integer ≤ x.

*** For an extra challenge, solve this problem without using a computer program.

No Solution Yet Submitted by K Sengupta    
Rating: 4.5000 (2 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
With only a little cheating | Comment 2 of 4 |
N/199 is a large number but its digits will eventually repeat.  So I started doing the long division by hand (just kidding I cut and pasted from Charlies post) to find that it repeats this 100 digit string:

3908431044109436069235064209938581797878280290340591848129536571747627024008933
55667225013958682300

these digits sum to 430 (I did do that by hand) and so the digital sum of the string is 7.

[N/199] is a 2008 digit number so it repeats the string 20 times with 8 extra digits left over: 39084310 These numbers have digital root 1.

So the answer is 20*7+1 = 141
1+4+1 = 6


  Posted by Jer on 2011-01-05 14:58:35
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