N=180 Aided by Excel only.

edited : I have erroneously started with N(0)=1991,

and continued : h(K)=mod(N(K),1991)

N(K)= MOD(10*h(K-1)+81),1991

So the value 0 is reached when N=182 (corrected answer).

Edited on January 29, 2016, 3:44 pm

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