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Math Help - Hi, I'm new to probability

  1. #1
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    Hi, I'm new to probability

    Hi,
    I'm totally new to this board and also to probability. I did A level pure mathematics with statistics and ever since I been really intrigued to work out probabilities of real life events .

    Now, suppose you have a person who is married and wanting to have a child. What is the probability that the child will be:-

    a) born on one's own birthday? and

    b) will be a girl?

    Both events happening in one's life.
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  2. #2
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    Quote Originally Posted by Probabilist View Post
    Hi,
    I'm totally new to this board and also to probability. I did A level pure mathematics with statistics and ever since I been really intrigued to work out probabilities of real life events .

    Now, suppose you have a person who is married and wanting to have a child. What is the probability that the child will be:-

    a) born on one's own birthday? and

    b) will be a girl?

    Both events happening in one's life.
    a) 1/365, assuming one's own birthday is NOT Feb 29.

    b) 1/2 (actually it's not quite ...... eg. Are There Any Factors That Can Influence The Probability Of Giving Birth To A Baby Boy Or Girl?)
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  3. #3
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    Quote Originally Posted by mr fantastic View Post
    a) 1/365, assuming one's own birthday is NOT Feb 29.
    And assuming the child's birthday is not Feb. 29! There is one Feb. 29 in 4 years or 1 in 4(365)+ 1= 1461 days. The probability you are born on Feb. 29 is 1/1461. The probability your child is born on Feb. 29 is also born on Feb. 29 is 1/1461 also so the probability you are both born on Feb. 29 is 1/1461^2. The probability you are NOT born on Feb. 29 is 1460/1461. The probability your child is not born on Feb. 29 and is born on the same day your were is (1460)/1461)(1/365) the probability you were not born on Feb. 29 and your child was born on the same day you were is (1460^2)/(1461^2(365)). The probability that your child was born on the same day of the year you were is the sum of those: 1/(1461^2)+ 1460^2/(1461^2(365))= 0.0027 which is, to 4 decimal places, exactly the same as 1/365= 0.0027.

    Sorry, I couldn't resist!

    I have read that baby boys are slightly more likely than baby girls but because boys (especially teenage boys!) have a higher death rate than girls, by age 20 the difference has disappeared.
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  4. #4
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    How wonderful, thank you both, really appreciate it

    Well actually the baby was not born on Feb 29th so I guess it's 1/365.

    Then I like to work the probability that it was both a) a baby born on one's birthday AND b) that it was a girl as well.... would that be just (1/365) x (1/2) ? = 0.000137

    Is that how you would work it for both those conditions to be true at the same time?
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