Friday, 2 February 2018

algebra precalculus - Finding the missing digit in a an Olympiad question.




The question is problem N0. 5



The question states that




229 is a 9 digit number with distinct digits. Which digit is missing?



(a.) 0(b.) 3(c.) 4(d) 5(e.) 7





The answer provided is a brute method and does not look very appealing. I tried to solve this without directly computing the number. Can anyone help me ??



My Attempt :



It is obvious to see that the number must include 0 and 9 , otherwise it would be divisible by 9 which cannot be true .



Using basic modular arithmetic , it also easy to show that



2292mod10




Hence the number also contains 2.



Similarly to our first observation , we may also conclude that the number must also contain 3 and 6 , otherwise it would be divisible by 3 , which is again not possible.



Hence the only missing digits could be (1,4,5,7,8).



But I cannot further deduce any new constraints . Is it possible to further reduce the number of cases ?


Answer



Following the suggestion by Lulu, we have that




2295(mod9)



wich is compatible with 4 missing indeed



(9i=0i)4=454=415mod9


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