Explanation for Question 24 From the Math (Calc) Section on the 2019 May Sat
So question 24 asks from 2005 through 2014, 2 the number of music CDs sold in the United States declined each year by approximately 3 15% of the number sold the proceeding year in 2005, 4 approximately 600 million CDs were sold in the United States of the following, 5 which best models see the number of millions of CDs sold in the United States 6 T years after 2005. 7 So if you look at the answer choices, 8 you'll see that all of the answers are in the same form, 9 and this is an exponential equation. 10 So there's a formula that you can use for exponential equations. 11 Um, and that's this formula. 12 So P is equal to a times one plus 13 or minus R to the T. 14 So in this formula a represents the initial value of 15 whatever you're changing. So initial value R represents 16 the rate at which you're either increasing or decreasing the initial 17 value and T represents time. 18 So in this case, the number of CDs is declining, 19 which means that we're going to use one minus R. 20 I means this is an exponential decay equation. 21 So all we need to do here is find a R and then T 22 we don't need to plug the value in four. We're just going to keep 23 it as two. So first let's find the initial value. 24 Well, originally there were about 600 million CDs. 25 So that means P is equal to 600 times 26 one minus R to the T. 27 And then the rate that it's declining at is 15%. 28 So 0.15. 29 So that means that this equation should be written as C is 30 equal to 600, which is the initial value times one minus 31 R not plus R. And it's also not just R one minus R. 32 So one minus zero point 15, 33 which becomes 0.85 to the T. 34 And so the answer to this question is B.