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Question:

n-digit numbers are formed using only three digits 2, 5 and 7. The smallest value of n for which 900 such distinct numbers can be formed, is?

6

9

7

8

Solution:

Clearly the number of such n-digit numbers is 3n as each place in the number can be filled by a non-zero digit 2, 5, 7. So we find the least n such that 3n > 900. Clearly n=7. So the correct answer is option D.