SivaKumarP
Hi Sabineodf,
Yes, you are correct. the answer is A and we need not to calculate. However, since I'm new to probability,after arriving at the answer for the data sufficiency, I just wanted to give a try in calculation to understand my strength in the probability sums. As I doubted, I got a doubt in doing that.
Using the 1st statement, I tried solving the problem
Method 1:
1 - (27/64) gives me 37/64
Method 2:
Katelyn wins the exactly one game = 1/4 * 3/4 * 3/4 =9/64
Katelyn wins exactly twice = 1/4 * 1/4 * 3/4 = 3/64
Katelyn wins exactly 3 thrice = 1/4 * 1/4 * 1/4 =1/64
If we sum up, we get 13/64
can you please tell me where I went wrong? Because I'm getting 2 answers for the same problem

Hi
SivaKumarP,
Let me tell why you have different answers from both the methods:
Method 1:P(Katelyn winning atleast 1 game) = 1 - P( Katelyn losing all the three games) \(= 1 - (3_{c_3} * \frac{3}{4} * \frac{3}{4} * \frac{3}{4}\)) = \(1 - \frac{27}{64} = \frac{37}{64}\) which is what you have calculated. Let's see your method 2 now.
Method 2:P(Katelyn winning atleast 1 game) = P(Katelyn winning only 1 game) +P(Katelyn winning only 2 games) +P(Katelyn winning all 3 games)
= (\(3_{c_1} * \frac{1}{4}* \frac{3}{4} * \frac{3}{4})+ (3_{c_2} * \frac{1}{4} * \frac{1}{4} * \frac{3}{4}) + (3_{c_3} * \frac{1}{4} * \frac{1}{4} * \frac{1}{4})\)
= \(\frac{27}{64} + \frac{9}{64} + \frac{1}{64} = \frac{37}{64}\) same as your answer in the first method.
You were getting a different answer because you did not choose combination of games which Katelyn would win or lose. So, when Katelyn wins only one game, she may win the 1st game or the 2nd or even the 3rd. Hence, there are 3 ways in which she can win 1 game. Same working would be applicable when Katelyn wins 2 games and all games.
Here method 1 should be the preferred way as it involves lesser no. of scenarios to calculate.
Hope its clear now!
Regards
Harsh
Awesome explanation. Many kudos..
One morething..3C1 means out of the 3 games she won 1?? Because I usually dont go with this formula as I used to go with some dash _ method. This formula seems to be new to me. can you confirm this one as well