A number of different seven digit numbers that can be written using on – InterviewSolution

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1.	A number of different seven digit numbers that can be written using only three digits 1, 2 & 3 under thecondition that the digit 2 occurs exactly twice in each number, is(A) 672(B) 640(C) 512(D) none of these
Answer» There are 7 total places. 2 is used only twice. Do select any 2 position. So 7c2 Remaining places have 2 possibilities each (1 or 3) So 2^5 TOTAL=7c2 * 2^5=672

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