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Bunuel
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Bunuel

GMAT CLUB TESTS' FRESH QUESTION:



Minimum of how many people are needed to have the probability of more than 1/2 that at lease one of them was born on either on Monday or on Tuesday?

A. 2
B. 3
C. 4
D. 5
E. 6


Say there are only 2 person..
Prob = \(\frac{2}{7}+\frac{2}{7}-\frac{2}{7}*\frac{2}{7}=\frac{4}{7}-\frac{4}{49}=\frac{24}{49}<\frac{1}{2}\)
Ans will be 3 because with 2, we had answer almost close to 1/2
No calculations required therefore
B
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chetan2u can you please explain a little bit more?

Posted from my mobile device
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Bunuel

GMAT CLUB TESTS' FRESH QUESTION:



Minimum of how many people are needed to have the probability of more than 1/2 that at lease one of them was born on either on Monday or on Tuesday?

A. 2
B. 3
C. 4
D. 5
E. 6

Assume no of people to be x. The probability of at least one of them is born on Monday or Tuesday is 1- the probability that all of them are born on rest of the days. The probability of one person to be born on the rest of the days=5/7. The probability that all of them are born on the rest of days=(5/7)^x. Required probability= 1-(5/7)^x, which, as per statement is greater than 1/2. So, 1-(5/7)^x=1/2 or (5/7)^x< 1/2, minimum x which satisfies this equation is 3. Hence, 3 or Option B is the answer.
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chetan2u can you please explain a little bit more?

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The reason for probability of atleast one of two person, say A and B, to be born on Mon or tuesday is..

So out of 7 days, we are taking just two days - Monday and tuesday..
Probability of A to be born on these 2 days = 2/7
Probability of B to be born on these 2 days = 2/7
But it is possible that both are born in these two days, so we have to subtract it once ..= 2/7 * 2/7 = 4/49
Probability = 2/7 + 2/7 - 4/49
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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!
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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)
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We shall use the complement principle because it is easier to calculate the probability of the opposite event instead of directly calculating the desired probability. The question asks for the probability that at least one person was born on Monday or Tuesday. Instead of counting all the ways this can happen, we first calculate the probability that no one was born on Monday or Tuesday and subtract it from 1.

Each person has a 5/7 probability of not being born on Monday or Tuesday. If there are n people, the probability that all of them avoid Monday or Tuesday is (5/7)^n. Using the complement rule, the probability that at least one person was born on Monday or Tuesday is:

1 - (5/7)^n > 1/2

We solve by testing values of n.

For n = 2, (5/7)^2 = 25/49 ≈ 0.510, so 1 - 0.510 = 0.490, which is less than 1/2.

For n = 3, (5/7)^3 = 125/343 ≈ 0.364, so 1 - 0.364 = 0.636, which is greater than 1/2.

Thus, the minimum number of people needed is 3.

The complement method is useful whenever a problem asks for "at least one" and it's easier to count the probability of the opposite event happening. It is commonly applied in problems involving birthdays, dice rolls, drawing cards, and defect rates in quality control.
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What if i approach like this

we need atleast 1 to have been born on monday/tuesday
and probability to be more than 1/2

if one of them is born on m/t in case of 2 people
the probability is 1/2
more than 2 solves?

this seems very vague but can someone counter me to help me understand

Bunuel
Official Solution:

Minimum of how many people are needed to have the probability of more than 1/2 that at lease one of them was born on either on Monday or on Tuesday?

A. \(2\)
B. \(3\)
C. \(4\)
D. \(5\)
E. \(6\)


The probability that a person is born on either on Monday or on Tuesday is \(\frac{2}{7}\) and the probability that a person is NOT born on either on Monday or on Tuesday is therefore \(\frac{5}{7}\).

Say minimum \(n\) people are needed to have the probability of more than 1/2 that at lease one of them was born on either on Monday or on Tuesday. In this case the probability that NONE of them is born on either on Monday or on Tuesday will be \((\frac{5}{7})^n\). Thus, the probability that AT LEAST ONE of them is born on either on Monday or on Tuesday will be \(1-(\frac{5}{7})^n\).

The question asks to find minimum \(n\) such that \(1-(\frac{5}{7})^n > \frac{1}{2}\):

\( \frac{1}{2}>(\frac{5}{7})^n\);

The least value of \(n\) that satisfies this is 3: \((\frac{5}{7})^3=\frac{125}{343} < \frac{1}{2}\).


Answer: B
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