FyodorPiketty
Can anyone explain why you can't just add 2/7 + 2/7 to test of the probability given a party of 2? (which is >1/2).
Why must we subtract (2^2/7^2) ? I'm not following why they can't both be born on Mon/Tues..
Thank you!
By your logic, think about what happens when we have 4 people. What is the probability that at least one of them is born on either Monday or Tuesday? You would say 2/7 + 2/7 + 2/7 + 2/7 = 8/7 (which is more than 1) but that is not possible. Probability can never be more than 1.
Think of Sets here. When we say probability that one person is born on M or T is 2/7, it includes the probability that the other person is born on any day including M and T. So the 'Both' part is double counted when we do 2/7 + 2/7. So you must subtract the probability that Both are born on M or T to get
2/7 + 2/7 - 2/7*2/7 = 24/49
OR
You can instead add these three: one is born on M or T and the other is not, the other is born on M or T but one is not, both are born on M or T
So 2/7 * 5/7 + 2/7 * 5/7 + 2/7 * 2/7 = 24/49
OR
You can find the probability that neither is born on M or T by calculating 5/7 * 5/7 = 25/49
So probability that at least one is born on M or T is simply 1 - 25/49 = 24/49 (same as before but much easier to find)
Hence, this is how I would solve it:
Probability that n people are all born on other 5 days = (5/7) * (5/7) * (5/7) ... n times
When n is 2, this becomes 25/49 (greater than 1/2)
When n is 3, this becomes 125/343 (less than 1/2)
Hence the probability with 3 people that at least one is born on M or T is 1 - 'less than 1/2' which will be more than 1/2.
Answer (B)