Posted by Maria on .
If 3 people are asked what day of the week they were born, find the probability that
i) 2 or 3 are the same.
ii) only 2 people are the same.
 is this 1/7 x 1/7 ??
If 4 people are asked what day of the week they were born, what is the probability that 2 or more people were born on the same day.
 How do I show this??
Using this idea, work out how many people need to be asked for their birthdate so that at least two people sharing their birthdate will have a chance of 1/2.
Thank you

Math 
PsyDAG,
i) It is the proportion below for 2 the same, plus 1/7*1/7*1/7 for 3 the same. (Eitheror probabilities are determined by adding the separate probabilities.)
ii) It is 1/7*1/7*6/7, because the third person has a different day of the week.
iii) Work the same as i) above, for 2, 3 or 4 sharing the same day.
The only way I know to find out is to expand iii) above to larger numbers until it equals 1/2.
I hope this helps. 
Math (Oops) 
PsyDAG,
Sorry, I was distracted by your attempt to answer ii). Your concern is for matching the first throw, whatever it is.
i) It is the proportion below for 2 the same, plus 1/7*1/7 for 3 the same. (Eitheror probabilities are determined by adding the separate probabilities.)
ii) The first throw can be any number, so your concern is with matching that number, so ii) = 1/7*6/7.
iii) Work the same as i) above, for 2, 3 or 4 sharing the same day.
The only way I know to find out is to expand iii) above to larger numbers until it equals 1/2.