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Worked solution

Exclude a digit using arithmetic sums

Find the sum of all positive integers less than 7070 which do not contain the digit 33.

CJC · Discussion · Question 3

Answer

18721872.

Subtract the two excluded arithmetic progressions

The total 1+⋯+69=69(1+69)/2=24151+\cdots+69=69(1+69)/2=2415. The integers 30,31,…,3930,31,\ldots,39 sum to 10(30+39)/2=34510(30+39)/2=345. The integers 3,13,23,…,633,13,23,\ldots,63 sum to 7(3+63)/2=2317(3+63)/2=231.

How did this part go?

Correct the double subtraction

3333 belongs to both excluded groups. Add it back once: 2415−345−231+33=18722415-345-231+33=1872.

How did this part go?

Common mistakes

The overlap 33 must be subtracted only once.

Read the hint