Kaplan Premier 2010 Edition - Is this a fair question?

[shaysmith]

On page 467, problem number 20 in data sufficiency, the book answer explanations indicates that since AD = AC we know that angle ADC = angle ACD, which is true. Then it goes on to say that we also know that angle ADE = angle ACB because they are supplementary angles, but I do not feel this is a fair assumption to make. The drawing appears to be symmetrical, but does not have to be. Line BE does not have to be a a straight line even though it looks like one. Therefore, I thought the correct answer to this questions should be E, but the book said the correct answer is A.

Does anyone have any thoughts on whether I am looking at this correctly or not? If I am thinking through this accurately, does the actual GMAT make these types of errors where assumptions are not given. I thought that the point of the GMAT is to not assume anything unless it is explicity given or can be derived from something given.

Any help is much appreciated!

Shay

[Stuart@KaplanGMAT]

On page 467, problem number 20 in data sufficiency, the book answer explanations indicates that since AD = AC we know that angle ADC = angle ACD, which is true. Then it goes on to say that we also know that angle ADE = angle ACB because they are supplementary angles, but I do not feel this is a fair assumption to make. The drawing appears to be symmetrical, but does not have to be. Line BE does not have to be a a straight line even though it looks like one. Therefore, I thought the correct answer to this questions should be E, but the book said the correct answer is A.

Does anyone have any thoughts on whether I am looking at this correctly or not? If I am thinking through this accurately, does the actual GMAT make these types of errors where assumptions are not given. I thought that the point of the GMAT is to not assume anything unless it is explicity given or can be derived from something given.

Any help is much appreciated!

Shay

Hi,

you've correctly identified a general guideline to follow in DS: don't make assumptions.

However, right in the directions for DS, it says:

"Lines shown as straight are straight..." (see p24 of the Official Guide, among other places),

so the question is set up just as it would be on the actual GMAT.

[shaysmith]

Wow, thank you very much for addressing that. I do see that now. It also says that lines that appear jagged are also straight. That's very interesting. Have you actually seen questions where you needed to make this assumption in order to get the correct answer; that a jagged line was actually straight?

[Stuart@KaplanGMAT]

Wow, thank you very much for addressing that. I do see that now. It also says that lines that appear jagged are also straight. That's very interesting. Have you actually seen questions where you needed to make this assumption in order to get the correct answer; that a jagged line was actually straight?

The "jagged lines are also straight" is there just to address the possibility of glitches in computer graphics (or poor screen resolution) - I've never seen a question that had an intentionally jagged line.