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Bunuel
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Bunuel
All digits, except the 44th digit (from the left), of an 80-digit positive integer N are 2. If N is divisible by 13, what is the 44th digit?

A. 1
B. 2
C. 3
D. 5
E. 6

Are You Up For the Challenge: 700 Level Questions

To solve this problem, please note that 222,222 is divisible by 13 (222,222/13 = 8,547).

We are given that N is an 80-digit number, with 79 of the 80 digits being 2. Now, we know the first 42 digits of 2’s are divisible by 13 (since these 42 digits consist of 7 groups of 222,222) and the last 36 digits of 2’s are also divisible by 13 (since these 36 digits consist of 6 groups of 222,222). Therefore, we are left with the 43rd and 44th digits. We know the 43rd digit is 2 and since 26 is divisible by 13, the 44th digit must be 6.

Answer: E

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When 222,222 divided 13 leaves remainder zero.
This cyclicity is applied till 42nd digit Of 80 digit no. N
Since whole 80 digit number N is divisible by 13
Take the cyclicity backwards. From 80 to 45
Where in every 6th interval of two no. The 6 digit no. 222,222 is divisible by 13.
This leaves only two no. To consider 43&44
Where 43 is 2 and 44 considerably X which is divisible by 14
20+x/13 leaves remainder zero.
Now consider multiple of 13 where 26 is the nearest multiple of 13 to 20+X
So X becomes 6.

OA is E

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Bunuel
All digits, except the 44th digit (from the left), of an 80-digit positive integer N are 2. If N is divisible by 13, what is the 44th digit?

A. 1
B. 2
C. 3
D. 5
E. 6

Are You Up For the Challenge: 700 Level Questions

222222 is divisible by 13. Till 42, all 2s are div by 13 (since cyclicity is 6, and 42 is divisible by 6). Ignore digits 43rd and 44th. Again from the 45th till 80th, there are 36 digits so all 36 2s are div by 13 (since cyclicity is 6, and 36 is divisible by 6). Remaining 43rd, this will be 2. From the options, only 6 fits because 26 is divisible by 13. So, correct answer is (E).
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Well a lot of people might not know the trick (like me), so we can start by trying to find the pattern in remainder.

After the sixth digit you will get zero remainder, from there 80 digit no. can be broken into set of six 2s.

7(6 2s)-----2 X-----6(6 2s)

from here it can be determined the value of X as 6.
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