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jmc

number theory intermediate

Problem

How many of the 200 smallest positive integers are congruent to 1 (mod 9)?
Solution
An integer congruent to 1 (mod 9) can be written in the form for some integer . We want to count the number of integers such that Subtracting 1 from all parts of the inequality, we get . Dividing by 9 we get . There are values of corresponding to positive integers from 1 to 200 inclusive that are congruent to 1 (mod 9).
Final answer
23