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
229≡2mod10
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
229≡5(mod9)
wich is compatible with 4 missing indeed
(9∑i=0i)−4=45−4=41≡5mod9
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