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Math Help - card shuffle

  1. #1
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    card shuffle

    Hello everyone!

    You have 9 cards with the one of the numbers 1-9 written on each and they are shuffled, you pick a card and write down the number and this is the tens digit of a 2-digit number, then you put the card back. You pick another card and this number is the units digit. so you might have a number like 27 if you pik a 2 then a 7.

    1. what's the probability of getting a number under 20?
    2. whats teh probability of getting a 14?

    thank you for reading my post
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  2. #2
    Bar0n janvdl's Avatar
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    Quote Originally Posted by Sweeties View Post
    Hello everyone!

    You have 9 cards with the one of the numbers 1-9 written on each and they are shuffled, you pick a card and write down the number and this is the tens digit of a 2-digit number, then you put the card back. You pick another card and this number is the units digit. so you might have a number like 27 if you pik a 2 then a 7.

    1. what's the probability of getting a number under 20?
    2. whats teh probability of getting a 14?

    thank you for reading my post
    1. Well then your first card must be a 1. The probability of that happening is \frac{1}{9}

    2. \frac{1}{9} to get a 1. and \frac{1}{9} to get a 4. Multiply them to get a probability of \frac{1}{81}

    EDIT: Sorry, i missed the part about "replacing" the card. Thanks Plato.
    Last edited by janvdl; January 6th 2008 at 12:39 AM.
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  3. #3
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    Quote Originally Posted by janvdl View Post
    2. \frac{1}{8} to get a 4.
    Think about that. Note the first card is put back into the deck.
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  4. #4
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    Hello, Sweeties!

    You have 9 cards with the one of the numbers 1-9 written on each.
    They are shuffled, you pick a card and write down the number
    . . This is the tens-digit of a two-digit number.
    You replace the card and pick another card.
    . . This is the units-digit of the two-digit number.

    1. What's the probability of getting a number under 20?
    2. What's the probability of getting a 14?

    You can pick any of 9 numbers for the tens-digit.
    You can pick any of 9 numbers for the units-digit.
    . . There are: . 9 \times 9 \:=\:{\bf 81} possible two-digit numbers.

    How many are under 20?
    . . Well, there is: . 11,\,12,\,13,\,14,\,15,\,16,\,17,\,18,\,19

    Get the idea?

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  5. #5
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    Quote Originally Posted by Soroban View Post
    Hello, Sweeties!


    You can pick any of 9 numbers for the tens-digit.
    You can pick any of 9 numbers for the units-digit.
    . . There are: . 9 \times 9 \:=\:{\bf 81} possible two-digit numbers.

    How many are under 20?
    . . Well, there is: . 11,\,12,\,13,\,14,\,15,\,16,\,17,\,18,\,19

    Get the idea?

    yes i think so, 9/81 = 1/9

    thank you all!
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