Jefferson School Sets - Matrix did not work

[vineetbatra]

Hello,

I tried to solve this question using the matrix table but cannot find the correct answer.

Thanks,

Vineet

The Matrix I build is as follows:

F NF Total
S ? ? 240
NS ? ? 60
Total 300 100 300

OA is D I get E

In Jefferson School, 300 students study French or Spanish or both. If 100 of these students do not study French, how many of these students study both French and Spanish?

(1) Of the 300 students, 60 do not study Spanish.

(2) A total of 240 of the students study Spanish.

[karmaresh15]

Hi Vineet ..

Please look at the sentence carefully. In Jefferson School, 300 students study French or Spanish or both.

The hidden information is that the number of students who don't read either spanish or french is 0.

Taking case I .. The matrix would be

Code: Select All Code


       F    not F     Total
S       
not S         0        60
Total        100       300


I think you can easily find out that number of students reading both french and spanish is 140.


Taking case II .. The matrix would be

Code: Select All Code


       F    not F     Total
S                      240
not S         0       
Total        100       300


Again you can find out that number of students reading both french and spanish is 140.

Hence in each of the cases above you can find out the required information. So, D

[vineetbatra]

Thats a great spot, I just completly missed that point.

Thanks,

Vineet

[karmaresh15]

Thats a great spot, I just completly missed that point.

Thanks,

Vineet

You are welcome :)

[esledge]

Great catch, karmaresh15.

The general rule is that the Double-Set Matrix is preferable to the Venn Diagram for 2 category scenarios. This is the only exception I've seen: 0 in the non-A/non-B category. In a Venn diagram, the 0 would go outside both circles, in the area most people ignore anyway.

[accounts]

I kind of disagree to

The hidden information is that the number of students who don't read either spanish or french is 0.

We cannot make such an assumption especially in DS.

S NOT S TOTAL

French x y

not F z w

they question is telling us x+y+z = 300 (In Jefferson School, 300 students study French or Spanish or both)

It does not clearly mention that w number of students do not study either.

On a problem solving such an approach is fine but not on DS.

[gokul_nair1984]

I kind of disagree to

The hidden information is that the number of students who don't read either spanish or french is 0.

We cannot make such an assumption especially in DS.

@accounts: You are wrong;you are not assuming anything but uncovering indirectly mentioned facts (especially in DS set theory and overlapping set theory questions)

It does not clearly mention that w number of students do not study either.

It has to be understood. They do not need to state everything explicitly; some information has to be alluded.

[RonPurewal]

I kind of disagree to

The hidden information is that the number of students who don't read either spanish or french is 0.

We cannot make such an assumption especially in DS.

S NOT S TOTAL

French x y

not F z w

they question is telling us x+y+z = 300 (In Jefferson School, 300 students study French or Spanish or both)

yeah, but, when you write the number 300 in the lower right corner of this matrix, you are committing yourself to only considering the three quantities represented by x, y, and z.

in other words, the entire universe of this problem consists of the students who take french or spanish (or both). we are free to ignore students who take neither, and thus to place a 0 in the corresponding box.