Explanation for Question 24 From the Math (Calc) Section on the 2019 October Sat
Now number 24, we're asked to use the systems of equations to 2 solve for three X plus three Y. And this will probably feel a little 3 bit unusual to you because normally we either solve for one variable. 4 It's not very often that we solve for both. And here they're asking us 5 to do even more work, right? Multiply it by three. 6 And so this question looks like it might take a really long time, 7 but in reality, this problem is already set up for it to make it 8 really easy for you to get these threes reason being what's the 9 difference between four and seven three. 10 And what's the difference between five and eight, also three. 11 So just knowing that, seeing that there's a way to manipulate these through subtraction 12 and addition to get the number three and also that, 13 um, we, again want the number three here. 14 We can probably use some sort of elimination version where instead of trying to 15 get rid of an X or get rid of a Y, 16 we're actually going to try to make it equal three X and three Y. 17 And so if we look here, we can see that if I do seven 18 minus four X, I get three X, 19 right. But right now I have a positive four. 20 So a good start might be to multiply this bottom equation by negative one 21 so that I can get a negative four X. 22 So I'll keep the top equation as is. 23 And if I multiply the bottom equation by negative one, 24 I get negative four X plus a Y is equal to negative nine. 25 Now let's add down and see what happens, right? 26 Seven minus ...