How many digits are needed to write 20! in decimal notation? : PS Archive
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# How many digits are needed to write 20! in decimal notation?

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How many digits are needed to write 20! in decimal notation? [#permalink]

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19 Sep 2006, 02:34
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How many digits are needed to write 20! in decimal notation?

(A) 16 (B) 17 (C) 18 (D) 19 (E) 20
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19 Sep 2006, 07:10
Yeah it is 19 (Checked on the calculator too )

Dunno if the approach is correct.
10! is around 3.6mn => 7 digits.
20! will definitely have at least 14.
Then every number after 15 should add another factor of 10, so (+5) = 19?

Is this method correct?
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19 Sep 2006, 07:10
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