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7s and 8s (Posted on 2024-11-20) Difficulty: 3 of 5
Find a 13 digit positive integer N whose base ten representation consists entirely of 7s and 8s such that 313 divides N.

No Solution Yet Submitted by K Sengupta    
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Some Thoughts computer findings Comment 1 of 1
list=[];
for n=0:8191
  ns=dec2bin(n,13);
  N=str2double(char(ns+7));
  m=mod(N,3^13);
  if ~ismember(mod(N,3^13),list)
    list(end+1)=mod(N,3^13);
  end
  % disp(mod(N,3^13))
  if mod(N,3^13)==0
    disp(N)
  end
end

length(list)
min(list)
max(list)
3^13-max(list)

checks all such numbers and finds no such divisible number.

The output 

ans =
        8100
ans =
   303
ans =
     1588566
ans =
        5757
        
shows there are 8100 different values mod 3^13, the smallest being 303 and the largest 1588566, which is 5757 short of being congruent to zero.

The numbers that lead to the lowest and highest mod values are  8877888777777 and 8788888878888.

  Posted by Charlie on 2024-11-20 13:45:56
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