at what minimum length of encoded letter blocks does the average number of binary characters of a prefix code a letter not exceed 1?

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Source without memory is generating letters $a,b,c$. $p(a)=\frac{25}{32}$, $p(b)=\frac{4}{32}$, $p(c)=\frac{3}{32}$. at what minimum length of encoded letter blocks does the average number of binary characters of a prefix code a letter not exceed 1?