# Thread: Hi, I'm new to probability

1. ## 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.

2. Originally Posted by Probabilist
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?)

3. Originally Posted by mr fantastic
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 $\displaystyle 1/1461^2$. 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 $\displaystyle (1460^2)/(1461^2(365))$. The probability that your child was born on the same day of the year you were is the sum of those: $\displaystyle 1/(1461^2)+ 1460^2/(1461^2(365))= 0.0027$ which is, to 4 decimal places, exactly the same as $\displaystyle 1/365= 0.0027$.

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.

4. 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?