A) 6
B) 7
C) 8
D) 9
Correct Answer: B
Solution :
Since at any place, any of the digits 2, 5 and 7 can be used, total number of such positive \[n\]-digit numbers are \[{{3}^{n}}\]. Since we have to form 900 distinct numbers, hence\[{{3}^{n}}\ge 900\Rightarrow n=7\].
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