double counting

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Huỳnh Anh Phương
28/08/2018 at 01:54
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Jason has $8. He going the bookstore to buy 2 books. If each book cost 1 dollar; 2 quarters; 3 dimes; 2 nickel; 4 cents, how much money he has left ?

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Huỳnh Anh Phương
19/08/2018 at 03:18
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Linda has 12 marbles. After play with Tony, she loses 3 marbles.

a) What fraction suppose number marbles Linda has left? Express your answer as a common fraction.

b) If in the first time Tony have 8 marbles, how many marbles will Tony have now?

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Huỳnh Anh Phương
12/06/2018 at 12:31
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Anna has 708 strawberries. She uses 12 strawberries to make cake, 20 strawberries to make juice. What percent Anna have left?
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    Nguyễn Mạnh Hùng 14/06/2018 at 02:10

    After using strawberries to make cake and make juice, Anna's strawberries left is: 

    708-12-20=676(strawberries)

    Anna is only has:

    676:708\(\approx0,95=95\%\)

    Huỳnh Anh Phương selected this answer.

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huynh anh phuong
31/05/2018 at 10:19
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If a and b are prime number , a is smaller than 110 and bigger than 99 and b is the greatest 1-digit number, whats is the total ?
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    Nguyễn Mạnh Hùng 14/06/2018 at 08:22

    According to the title, we have:

    99<a<110 and a is a prime number

    So a=103

    b is the greatest 1-digit number so b = 9

    a+b=103+9=112


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huynh anh phuong
12/12/2017 at 10:59
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There are many people in the party, in the party has 4 tables, each tables have 3 plates, each plates has 9 pancakes. How many people in the party?
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    Lê Quốc Trần Anh Coodinator 29/12/2017 at 18:00

    The title is missing some information: Each person has a pancake.

    So there are: \(4.3.9=108\left(people\right)\) in the party.

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    FA Liên Quân Garena 30/12/2017 at 21:44

    The title is missing some information: Each person has a pancake.

    So there are: 4.3.9=108(people)

     in the party.

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    nguyen tuan anh 06/04/2018 at 13:48

    the title is missing some information : Each person has a pancake

    so there are : 4.3.9 = 108 (people) in the party


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Đỗ Phi Phi
26/07/2017 at 21:39
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A prime number is called a jillyprime when doubling it and adding 1 results in another prime. How many numbers lass than 15 are jillyprimes? (Note that 1 is not a prime.)
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    Phan Thanh Tinh Coodinator 26/07/2017 at 23:34

    2 x 2 + 1 = 5

    3 x 2 + 1 = 7

    5 x 2 + 1 = 11

    7 x 2 + 1 = 15

    11 x 2 + 1 = 23

    13 x 2 + 1 = 27

    So,there are 4 jillyprimes which are less than 15 : 2 ; 3 ; 5 ; 11 

    Selected by MathYouLike
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    FA KAKALOTS 28/01/2018 at 22:04

    2 x 2 + 1 = 5

    3 x 2 + 1 = 7

    5 x 2 + 1 = 11

    7 x 2 + 1 = 15

    11 x 2 + 1 = 23

    13 x 2 + 1 = 27

    So,there are 4 jillyprimes which are less than 15 : 2 ; 3 ; 5 ; 11


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Hà Đức Thọ moderators
14/06/2017 at 15:35
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A house has 10 rooms. Ten boys stay in different rooms and count the number of doors in them. After that they sum all results and receive 25. What a proposition can't be true about number N of doors which led outside the house?
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    tth 30/10/2017 at 15:14

    We have:

    25 do not divide to 10

    So 25 can't be true about number N of doors .

    Note: I'm not certain

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    Ma Kết 05/04/2018 at 14:42

    We have:

    25 do not divide to 10

    So 25 can't be true about number N of doors .

    Note: I'm not certain

  • ...
    Ma Kết 05/04/2018 at 14:41

    Hà Đức Thọ            


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falcon handsome moderators
09/03/2017 at 12:34
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Let the answer be k . We count the total number of paris of students were on duty together in the k days. Since every pair of students was on duty together exactly once, this is equal to \(C^{15}_2\) x 1 = 105. On the other hand, since 3 students were on duty per day, this is also equal to \(C^3_2\) x k = 3k. Hence 3k = 105 and so k = 35