Explanation for Question 18 From the Math (Calc) Section on the 2020 October Sat
So question 18 is asking us to take our units, 2 which are in millions of feet per second, 3 and trying to change it to yards per minute. And so any question that's 4 asking you specifically to change units is what we call a unit analysis question. 5 Um, and so here, we're starting off with three, 6 320 million feet over one second. 7 And so the first thing we can think about doing is trying to convert 8 that feet to yards and they give us a, 9 um, a proportion here. 10 One yard is three feet. And so if I want to get rid of 11 the feet, right, I'm trying to get rid of the feet and put yards 12 into my equation. Then I want this feet that I have here to cancel. 13 So whatever I'm trying to get rid of here, as far as the feet, 14 I put it in the opposite place of where it is. 15 For example, feet is in the numerator here. 16 So when I write my second a fraction, 17 I want to put three feet in the, in the denominator and then the 18 one yard and the top, that way, 19 when I multiply it across my feet, cancel, 20 then we can think about trying to get rid of the seconds. 21 Now, here seconds are on the bottom. So when I write my next fraction, 22 that's going to get rid of the seconds. I'm actually going to put seconds 23 in the numerator. So that way they still cancel. 24 And I'm trying to get into yards per minute, 25 right? And so we know that there are 60 seconds in one minute. 26 Now we can see these seconds would cancel because one is in the numerator 27 and one is in the denominator and the new remaining units that I'm 28 left with because the seconds have canceled. 29 The feet have canceled. All I'm left with is yards per minute. 30 And that was what I wanted. So from here, all I have to do 31 is multiply across. So if you do 320 million 32 divided by three times 60, 33 you get about 64,000 million giving 34 us the correct answer of C.