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anonymous

  • 5 years ago

how many positive integers are there less than 800 such that all the digits are odd?

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  1. anonymous
    • 5 years ago
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    hey, how's it going?

  2. anonymous
    • 5 years ago
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    good

  3. anonymous
    • 5 years ago
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    so let's start with 1,3,5,7,9.

  4. anonymous
    • 5 years ago
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    ok

  5. anonymous
    • 5 years ago
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    that is a good base for us, because it's good to remember that there are 5 possibilities for the end digit

  6. anonymous
    • 5 years ago
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    why

  7. anonymous
    • 5 years ago
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    now what can the 2nd digit be? 1,3,5,7,9, of course

  8. anonymous
    • 5 years ago
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    what i dont understand

  9. anonymous
    • 5 years ago
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    so for up to 100, we have the 5 1-digit numbers, plus 5 possibilities for the tens digit, and there are 5 possibilities for the ones digit for each of those. so that's 25 more.

  10. anonymous
    • 5 years ago
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    you understand so far? in the 10s, 11,13,15,17,19 work. that's 5. in the 30s, 31,33,35,37,39. 5. so there's a pattern. 5+5+5+5+5 = 25 numbers that have all odd digits from 10 to 99

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