The function f is defined for each positive three-digit integer n by 
, where x, y and z are the hundreds, tens, and units digits of n, respectively. If m and v are three-digit positive integers such that 
then 
?
(A) 8
(B) 9
(C) 18
(D) 20
(E) 80
The real trick here is to think about f(m) and m separately. Consider the digits of m as X, Y, Z for the hundreds, tens, and units, etc. Once you write out X = 2, Y = 3, Z = 1 (for example) as 231, then it becomes a bit simpler.
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