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Welcome , TinyCooker !

the -th string is 1,2,3,4,... , n
So the 11th string is 1234567891011
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The correct explanation is as follows :

When the string is merged to 200 strings then somethings happens like this -

1st string : 1 : 1 digit
2nd string : 12 : 2 digit
4th string : 123 : 3 digit
so 200th string means : 1+ 2 + 3 + .......n = 200
so we need to find n, after solving the above equation we get a zist that n should lie in between 19 and 20.

so let put n =20 in the AP sum formula :

Sn = (n/2) (2a + (n-1)d)
so S19 = 190

hence we need 10 more that means, 1,2.3....(0)

hence 0 is the correct answer
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Lengths of the strings are:
1, 2, 3, ..., 9, 11, 13, 15, 17, 19, 21, 23, 25, ...
The twist is that after the 9th string, two digits will be added instead of one.
First 9 strings contribute:
1+2+...+9 = 45 digits.

Now add the next lengths:
If I take 10th to 17th string, it contributes to
11+13+15+17+19+21+23+25 = 144 digits
So up to the 17th string, the total is:
45 + 144 = 189 digits
Therefore, the 200th digit is in the next string, which contains 27 digits.
123456789101112131415161718
Since 189 digits have already been used, the 200th digit is the 11th digit of this string.
Therefore, the 200th digit is 0.
Answer: A
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