Answers

2014-06-01T21:24:06+08:00
An = Ai + D(N-1)

starting with two 

30 = 12 + 2N - 2 
20 = 2N 
N = 10 

12 - 30 = 19 digits 

19 - 10 = 9 

9 digits are not divisible by two 

then by three 

30 = 12 + 3N - 3
21 = 3N 
N = 7 

7 - 4 = 3 since 4 digits that are divisible by three are already divisible by two. 

so 9 - 3 = 6 

6 digits are not divisible by two nor three 

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