shapla wrote:

How many of the students in a certain class are taking both a history and a science course?

1) Of all the students in the class, 50 are taking a history course

2) Of all the students in the class, 70 are taking a science course

Dear

shapla,

I'm happy to respond.

Given the prompt, before we get to the statements, think about everything we

don't know. We don't know how many students are taking history, or are taking science, or are in the class as a whole.

It's probably apparent that neither statement by itself is sufficient to answer the prompt question. Let's consider them together.

Suppose we know that

(1) 50 students are taking history

(2) 70 students are taking science

and we want to know about the overlap, the folks taking both. We still don't know the class size.

Consider these two scenarios:

SCENARIO #1: Class size = 70

In this scenario, everyone is taking science, so the 50 who are taking history are necessarily taking it in addition to science. In this scenario, those 50 would be the overlap, the number of students taking both.

SCENARIO #2: Class size = 1000

An extraordinarily large class size, perhaps typical of a university. Here, if we picked 50 people at random and assigned them to history, then picked 70 people at random and assigned them to science, we would be likely to get few overlaps, if any. It would be easy for one set of 50 to take history, and other set of 70 to take science, leaving 880 students unenrolled in either. So, there may be no overlap, or there may be some handful of students enrolled in both. We have no way of determining.

One scenario led us to one answer. Another scenario left the prompt question unanswered. We would only have sufficient information if every imaginable scenario produced the same answer to the prompt question. Given the information we have, even with both students, we have absolutely no hope of answering the prompt question. Even with both statements combined, the information is

insufficient.

Answer =

(E) My friend, if GMAT DS is relatively new to you, you might find this blog article helpful:

http://magoosh.com/gmat/2013/gmat-data- ... ency-tips/Please let me know if you have any questions about what I have written here. Best of luck, my friend!

Mike

_________________

Mike McGarry

Magoosh Test Prep

Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)