Explanation for Question 24 From the Math (Calc) Section on the 2020 March Sat
Now, we're looking at question number 24, 2 which gives us a table about the change in value of 50 stocks. 3 And it tells us that the two way table categorizes them in July and 4 August. Right? And if one of the, 5 here's the question, here's the important part. Now, if one of the stocks that 6 increased in value in August is chosen at random. 7 So that's the first part. And then it asks us what is the probability 8 that the stock also increased in July? 9 So what we're doing here is we can pretend that there are different groups 10 of stocks. And we are looking only at the group of stocks that 11 increased in value in August. 12 So right here, this is the group that we're interested in. 13 This total of 30 stocks increased in value in August. 14 And those are the ones that we're going to choose from. 15 So what our denominator should have for our probability is 30 because the total 16 stocks that we're looking at is not all of the 50 of them, 17 but rather only the ones that increased in value in August. 18 So now we can actually just forget about the rest of this table. 19 We're going to pretend that the only row that exists on this table is 20 the one that I've highlighted in blue. And now it's asking of those 30 21 stocks, what is the probability that it also increased in value in July? 22 So now we're going to focus on this box, 23 right? 21 of them increased in value in July. 24 So 21 out of 30 increased in value. 25 And when you plug that into your calculator, you'll g...