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Problem #62 · September 27, 2026

Natural numbers

Find the sum of the digits of the following number. 1234567891011…19201234567891011 \ldots 1920
  1. A102102
  2. B100100
  3. C9090
  4. D110110

WORKED EXAMPLE

Solution

This number is formed by writing the numbers from 11 to 2020 consecutively. The digits 1..91..9 sum to 4545; in the numbers 10..1910..19, the tens place contains ten occurrences of 11 (1010) and the units place contains the digits 0..90..9 (4545) — a total of 5555; 2020 gives 2+0=22+0=2. Altogether: 45+55+2=10245+55+2=102.
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