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Current time:0:00Total duration:10:35

GMAT: Data sufficiency 1

Video transcript

let's do some data sufficiency problems so the first problem problem number 1 on page 278 it says how much is 20% of a certain number so we want to know what 20% of a number let me just call that X so what is that so the first data point they give us is that 10% of the number is 5 so we could say that is 10% times X is equal to 5 another way we could just write that is point 1 X is equal to 5 and then the second data point I guess we could call it is 40% of twice the number is equal to 40 so 40% 0.4 times twice the number so times 2 X right we chose X as our number is equal to 40 so what we have to figure out is do either of these data points allow us to figure out extra - maybe we need both of them or maybe we can't figure out even with all of this information well this is a simple a simple linear equation right very easy way to think about it is you could multiply both sides of this by 2 both sides equation and you end up with 0.2 times X is equal to 10 well that's the same thing as saying that 20% of X is equal to 10 so statement 1 alone is is all we need to figure out this let's see if what statement 2 gets us so if we simplify this expression just a little bit we get let's see point 4 times two we have 0.8 times X is equal to 40 and here instead of multiplying both sides of this equation by 2 we could divide both sides by 4 right because we want to get 20 percent of X this is 80% of X so if we divide both sides of this by 4 we get 1 we get point 2 X is equal to ten there you go 20% of X is equal to ten so either of these alone are enough to solve that problem and I always forget what the letters aren't but I think that is I think that is statement well I'll leave it for you to figure it out I know there's a statement that's statement D each statement alone is sufficient all right problem number two I don't want to waste too much time on that problem number two and I'm trying to do this in real time without looking at the answers because I really want you to get a sense of how someone thinks about this if they've never seen the problem before and so I'll ask you to bear with me a little bit because maybe I'll get a problem wrong and maybe that'll be instructive if I do okay they say a thoroughly blended biscuit mix includes only flour and baking powder flour and baking powder baking powder what is the ratio of the number of grams of baking powder to the number of grams of flour so we want to know this if we want another ratio of baking powder to flour that's what they we need to figure out now statement number one they tell us exactly nine point nine grams of flour is contained in ten grams of the myth mix so they're nine point nine flour how much and there's exactly ten how much how much baking powder is there well baking powder is going to be equal to 10 minus the amount of flour so it's going to be what point 1 grams right nine point we could say that that is equal to flour so we could easily just using this first statement figure out the ratio write the ratio of baking powder to flour is 0.1 we just took 10 minus the amount of flour 29.9 or you could say it's 1 to 99 whatever however you need it but that's what's fun about this film you don't have to figure it out we just have to say we could figure it out so 1 is enough by itself let's see what statement 2 tells us statement 2 and I'll do it in a slightly different color statement 2 says let me scroll down a little bit statement two says exactly 0.3 gram I think I think that's an error in the book it should be grams exactly 0.3 gram of baking powder is contained in 30 grams of the mix so if we have 0.3 baking powder baking powder that's grams so and it says that tells us there's 30 grams of the mix so how much flour do we have well it's 30 30 minus 0.3 is equal to what is equal to twenty nine point seven twenty nine point seven flour and so once again we can easily figure out the ratio of baking powder to flour its 0.32 twenty nine point seven which is once again one two ninety nine but we didn't even have to figure that out we just know that this is enough so once again we know that D each statement alone is sufficient unless I miss something all right problem number three problem three let me scroll down a little bit what is the value of the absolute value of x so they want to know the absolute value of X is equal to what and the first statement they tell us the first statement they tell us is that X is equal to the minus absolute value of X so what what numbers is this true for that the number is equal to the minus absolute value it's definitely not true for positive numbers right I'm going to put a positive one there one is not equal to the absolute value of 1 is 1 so 1 is not equal to negative 1 this works for 0 right 0 is equal to negative 0 is equal to 0 and this works for negative numbers try it out negative 1 right negative 1 is equal to the negative absolute value of negative 1 well the absolute value negative 1 is 1 so you get negative 1 is equal to negative 1 so it works for 0 and negative number so that all this tells us and maybe maybe this will help us in conjunction with the second part who knows this just tells us that X is less than or equal to 0 it's just a convoluted way of saying this let's see what it where that gets us now the second statement second statement let's see they say that x squared is equal to four x squared is equal to four well this tells us that X is equal to plus or minus two now if we were if we were trying to figure out what X is equal to we would actually need both of these statements because this statement says oh X is positive 2 or negative 2 and then this statement says that oh X is definitely negative so if we were trying to figure out what the value of x was we would need both of these statements but notice they're asking us what's the value of the absolute value of x all right so regardless of whether X is positive or negative 2 the absolute value of either positive or negative 2 is always going to be equal to 2 so really this is all you need you only need statement two to figure out this problem and so that is B you only need statement two all right I think I got that one right let's see problem number what am i on problem number four I'll do it in this brownish color problem number four is R greater than 0.2 seven very simple question is are greater than 0.2 seven they asked ok statement one tells us that R is greater than one-fourth okay well that doesn't really ya mean R could still be you know this is 0.25 so R could still be equal to 0.25 one right it could be 0.25 1 or it could be 0.3 so this really doesn't give us any information that we don't know whether R is greater than 0.2 7 let's see the second statement tells us second statement says R is equal to 3/10 R is equal to three tens well that is equal to 0.3 and that's definitely greater than 0.2 7 so this is all we need we just need statement to statement 1 was a little useless so and that is I always forget what letters that's B we only need we only need statement - all right how am i doing on time oh I have time I think I can do a couple more problems these go fast you don't have to actually do the math problem number five what is the value of the sum of a list of n odd integers so sum of n odd integers okay and they didn't say consecutive and they didn't say where it starts it's just an odd integers fair enough okay one they say tell us that n is equal to eight so if I knew that I was going to sum eight odd integers can I know the sum well I know I mean it though it could be you know these all of these could be the numbers in the millions right there or they could be you know negative well or they could be you know really small numbers so that doesn't help me much point number two they say the square of the integers on the list is 64 so they're essentially saying that N squared is equal to 64 and since we're talking about numbers and lists it can't be negative so n is equal to 8 so these are really telling me the same thing just this is a little bit more confident but really they don't help out I mean I could have eight numbers in the billions or I could have eight numbers in in the hundreds and so I'm obviously going to get a different sum so I don't think I can figure this out anyway so that's what that is statement e o that's e that they're together are not sufficient so that's so neither both of these are useless I still don't know the answer which is e all right well I think I'm out of time I just passed the 10-minute mark I will continue in the next video see you soon