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A function $f$ maps the positive integers to the nonnegative integers such that

(i) $f(r s)=f(r)+f(s)$ for all $r, s$,

(ii) $f(n)=0$ if the last digit of $n$ is 3 ,

(iii) $f(10)=0$.

Find $f(1985)$ and justify your answer.
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