Explanation for Question 9 From the Math (Calc) Section on the 2019 April Sat
For number nine, we've requested on percentages. 2 So on November 1st, there were 2,500 boxes in a warehouse on December 1st, 3 there were 15% fewer. And then on January 1st there were 20% more. 4 And we want to know how many boxes were in the warehouse on January 1st. 5 So a common mistake, I can see a student making on this one is 6 seeing that there were 15% fewer, 7 and then 20% more makes you think that there's going to be 5% more 8 in the end. So if you take 2,500 and multiply by 5% increase, 9 that gives you D but D is not the correct answer. 10 And that's because percentages don't work like that. 11 So think about it like this. If I was in a store and I 12 wanted an item that was $10, 13 and let's say the store was having a 50% off sale. 14 So that means it would be $5. 15 Let's say I had a 50% off coupon on top of the 50% off. 16 That doesn't mean that I'm getting my item for free. 17 It means that I'm getting 50% off of the 50% that was already taken 18 off. So that means I'm going to pay $2 and 50 cents. 19 Since I took 50% off of the price that was already cut in half. 20 So we have to do the same thing here. 21 You have to go step-by-step you can't just subtract the percentages and then jump 22 to the end. So if we start off with 2,500 boxes, 23 first, let's find how many there were on December 1st. 24 So to find that you take your original number 2,500 25 and multiply by one plus or minus 26 the percentage. And now we determine which sign 27 we're using based on...