Each of the students in a certain class received a single grade of P

LM

I started to use Venn but there was no point IMHO.

We don't know what P or F or I represent. no proportion of the total class was given.

does 40% of P represent 10% of the entire class? 90% of the class?

what does 20% of F or I represent of the total percentage of the class? we dont know. maybe my reasoning is wrong, but that is how I came up with my answer: E

So taking 1&2 together we know 80% of those with a fail/incomplete were male so 20% with F&I were female correct? If 40% of those who passed were female, then we can determine that 30% of the students were female.

This was my rationale towards an answer of C. Can anyone explain where I went wrong?

Yup, its E. We can't assume that P,F or I are equally weighted.

Takeaways: This one was a bit tricky because I approached the problem as a double matrix set but it had 3 variables in the columns and 2 variables(male/female) in the rows. I also attempted to do percentages that lead up to a total but it was getting more complex than I would like. What has helped was recognizing this as a weighted average problem. Therefore an easy method could be:
Each of the students received a single grade so
T (total) = F + P +I
Since we have male and female : T=(M)F+(F)F+(M)P+(F)P+(M)I+(F)I or organize like this T=(M)F+(M)P+(M)I+(F)F+(F)P+(F)I
Translating the statement word by word I got (W/100)T= F? which is the main question we are looking to answer but made it easier on myself by just writing (F/T)*100%=?
1. Of those who received a P, 40% were females
This is clearly insufficient because (F)(P)= .4P and (M)(P)= .6P does not give us any actual numbers to plug in and we still need the other weighted averages.

2. Of those who received either an F or I , 80% were males.
T=.8F+.2F+ (M)P+(F)P+(.8)I+(.2)I
Insufficient because we need actual numbers because weighted averages depend on quantities

(1) and (2)
Insufficient because although we have several percentages we still need quantities because we can have 80/100 total students in f and I be males but then have 4/10 students in P be women.

Bunuel - thoughts?

Hi GMAT01,

You ARE correct. When dealing with these type of broad percent-based questions, it's important to be a bit cynical. You do NOT know how many students are in each subgroup, so it would take a LOT of information to answer this question.

As an example, when Fact 1 tells us "Of those who received a P, 40 percent were females", we don't know if 5 people got a P (in which 2 of them were female) or 500 people got a P (in which 200 of them were female). Making the individual group numbers relatively big or relatively small will impact the overall calculation. This can save you some time and effort.

GMAT assassins aren't born, they're made,
Rich

@Bunnel - Is it safe to say that in these type of questions until we get the entire class strength in either of the statements, we would always choose E. As stated in your example, we are plugging different set of values and getting different set of answers.

Thanks.

This is a weighted average question.

(1) Females make up 40% of group 1 (P)
(2) Females make up 20% of group 2 (F or I)

Together, Female will make up somewhere between 20% and 40% of the whole population. The exact percentage depends on the ratio of group sizes. If there are twice as many people in group 1, then the percentage will be twice as close to group 1 datapoint (40%) as it is to group 2. On the other hand if there are three times as many people in group 2, then the percentage will be three times as close to group 2 datapoint (20%) as it is to group 1. Consider below scenario :-

Let T = total number of students in the class

Each received a single grade so T = F + P +I

? % of T = Females

1. of those who received a P, 40% were females

it doesnt give us the the exact number

2. of those who received either an I or I(I or I ??..may be one of the other two) , 80% were males.

Still it doesnt give us the number of females

so E

We are given that each of the students in a certain class received a single grade of P, F, or I. We can let p = the number of students who received P, f = the number of students who received F, and i = the number of students who received I. Thus, the total number of students in the class is p + f + i. We need to determine the percentage of students who were females.

Statement One Alone:

Of those who received a P, 40 percent were females.

This means 0.4p students are females. However, since we know neither the values of p, f, and i nor the percentage of students who received an F or I, statement one alone is not sufficient to answer the question.

Statement Two Alone:

Of those who received either an F or I, 80 percent were males.

This means 20 percent of the students who received either an F or I were females. In other words, 0.2(f + i) students are females. However, since we know neither the values of p, f, and i nor the percentage of students who received a P, statement two alone is not sufficient to answer the question.

Statements One and Two Together:

From the two statements, we can say that the percentage of students in the class who were females is:

(0.4p + 0.2(f + i))/(p + f + i) x 100

However, since we don’t know the values of p, f, and i, we can’t determine the numerical value of the expression above. Statements one and two together are still not sufficient.

Answer: E

Is this approch fine?
P = .4 F so .6 M
F+L = .8M so .2F

.4F+.2F\ (.6M+.4F)+(.2F+.8M)

Even after cancelling F we dont know M so not sufficiant.

Making a chart:


Even if we assume T=100 (which we could do since we're not given any concrete numbers and the problem asks for a %), we can't determine the breakdown of 100 between P, F, I or between Male+Fem without more information.

Target question: What percent of the students in the class were females?

Let's use the Double Matrix Method.
This technique can be used for most questions featuring a population in which each member has two characteristics associated with it (aka overlapping sets questions).
Here, we have a population of students, and the two characteristics are:
- male or female
- received a P or received an F or an I

Since the target question asks for a PERCENT, let's say there are 100 students in total.
So, in order to answer the target question, we need to determine the number of females in the group.

We can set up our matrix as follows:


At this point, we should recognize that statement 1 does not provide any information about the male/female split of students who received an F or an I.
So, statement 1 cannot be sufficient.

Likewise, statement 2 does not provide any information about the male/female split of students who received a P.
So, statement 2 cannot be sufficient.

Statements 1 and 2 combined
Even when we combine the two statements, we realize that we're not told the number of students who received a P and the number of students who received an F or an I
So, there are many possible scenarios that satisfy BOTH statements. Here are two:

Case a: It COULD be the case that 50 students got a P, and 50 students got an F or an I.

In this case, the answer to the target question is 30 percent of the students are female

Case b: It COULD be the case that 90 students got a P, and 10 students got an F or an I.

In this case, the answer to the target question is 38 percent of the students are female

Since we cannot answer the target question certainty, the combined statements are SUFFICIENT

Answer: E


This question type is VERY COMMON on the GMAT, so be sure to master the technique.

To learn more about the Double Matrix Method, watch this video



Here's a practice question too!

Treat this as a weighted average problem.
Statement 1 => A "mixture" with 40% Female
Statement 2 => A "mixture" with 20% Female

Individually, each one is insufficient.

(1) + (2) => Yields a "mixture" that comprises the entire population (P+F+I).
Now, in order to tell the percent of females in the resultant "mixture", we need to know the proportions in which the 2 "mixtures" were added - ie, the percent of P and/or percent of (F+I) in the entire population.
Since we don't have that information, choose E.

Similar Question: https://gmatclub.com/forum/each-of-the- ... 48323.html

What we need:
Option 1 - the sum of the percentage of female in P, F, I and their weights
Option 2 - number of females in P, F, I / the total number of students

From (I) & (II), we know the percentage of female in P & F+I. However, we don't know the weights of P, F and I. Therefore, we cannot answer the question.

Ans: E

Bunuel
I see that you grouped F and I as one category... is this because "either" means to add the categories together?
I was disproving this statement by also thinking that "F" and "I" could take on various values as well. Thank you!

Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.