birthday problem

M

mst_1026

New member
Joined
Mar 5, 2021
Messages
7
if you were born on Monday, what is the probability that you were born on the same day of the week as one or both of your friends??

i am having a difficult time fully constructing the equation together. would you just do 1/7*1/7*1/7
 
mst_1026 said:
if you were born on Monday, what is the probability that you were born on the same day of the week as one or both of your friends??

i am having a difficult time fully constructing the equation together. would you just do 1/7*1/7*1/7
Click to expand...
I would work t backwards!

What is the probability that both your friends will "miss" your birthday - to be born?
 
mst_1026 said:
if you were born on Monday, what is the probability that you were born on the same day of the week as one or both of your friends??

i am having a difficult time fully constructing the equation together. would you just do 1/7*1/7*1/7
Click to expand...
If you WERE born on Monday then the 1st 1/7 should be a 1. Is that clear? What about the other 1/7s?
 
Jomo said:
If you WERE born on Monday then the 1st 1/7 should be a 1. Is that clear? What about the other 1/7s?
Click to expand...
that is clear, however, i am still unsure on how to construct the equation. this seems like rocket science
 
Whoa. Probability theory is hard because the formulas are idiotically simple, but figuring out which formulas to use requires hard thinking.

If you were born on a Monday, what is the probability that you were born on a Monday?

If you were born on a Monday, what is the probability that Friend A was also born on a Monday. What is the probability that Friend A was not born on a Monday but friend B was born on a Monday.
 
I would asked the following 2 part question on an exam for every probability course that I taught.

a) If you had 5 children, then what is the probability that exactly 3 of them were boys.
b) If you had 5 children, then what is the probability that exactly 2 of them were girls.
Almost everyone got the correct answer. However those who got it correct, showed detailed work for both parts.
 
Jomo said:
I would asked the following 2 part question on an exam for every probability course that I taught.

a) If you had 5 children, then what is the probability that exactly 3 of them were boys.
b) If you had 5 children, then what is the probability that exactly 2 of them were girls.
Almost everyone got the correct answer. However those who got it correct, showed detailed work for both parts.
Click to expand...
@Jomo

I am not sure how I react to that anecdote. In one sense, I am ecstatic that most students got it right. On the other hand, I am despondent that most had no clue how to proceed efficiently. Maybe this is why most teachers I have known are heavy drinkers.
 
mst_1026 said:
if you were born on Monday, what is the probability that you were born on the same day of the week as one or both of your friends??

i am having a difficult time fully constructing the equation together. would you just do 1/7*1/7*1/7
Click to expand...
What is the probability that freind1 was NOT born on your birthday? = 6/7

What is the probability that freind2 was NOT.born in your birthday? = 6/7

What is the probability that friend1&2 were NOT born on your birthday? = 6/7 * 6/7

Continue....
 
There are frequently several ways to solve problems in probability. One very wise helper here says that he is never confident that he has correctly solved a problem in probability until he has got the same answer two different ways.

[MATH]\text {P(A) + P(not A)} = 1 \implies \text {P(A)} = 1 - \text {P(not A)}. [/MATH]
That is one way.

[MATH]\text {If X and Y are independent, P(X or Y) = P(X) + P(Y)} - \text {P(X) * P(Y)}.[/MATH]
That is another way.

How can you define event A to suit your problem?

How can you define events X and Y to suit your problem?
 
You must log in or register to reply here.
Top