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How many decimal numbers 0-4095 can be converted into a binary number that has exactly 2 '1s'?

Pseudonym
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1 Answers1

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Hint. Note that $4095=2^{12}-1$, therefore the integers in the interval $[0,4095]$ are precisely the ones that can be represented by using 12-digits binary numbers. Now among the 12 digits you have to choose 2 digits where to place the ones. In how many ways this choice can be done?

Robert Z
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