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Data Sufficiency (DS) Traps [#permalink]
05 Apr 2008, 02:44

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Hi friends,

I open this thread because, I think, recognizing a pattern is very helpful in Math, expecially in DS with a very tricky problems. When we are in the battle, time is a gold.

In one long post of Traps Collection of the forum, I found that there was no general discription to differentiate the trap called SPY GIRL. So, if you dont mind, let paste here any problem has E choice as correct one. From that, may we have a rule? what do you think? I am first.

I. SPY GIRL 1. What fraction of this year's graduating students at a certain college are males?

a. Of this year's graduating students, 33 percent of the males and 20 percent of the females transferred from another college. b. Of this year's graduating students, 25 percent transferred from another college. AC=E

II THE TWIN 1. Martha bought an armchair and a coffee table at an auctio and sold both items at her store. Her gross profit from the purchase and sale of the armchair was what percent greater than her gross profit from the purchase and sale of the coffee table?

a. Marth paid 10 percent for the armchair than for the coffee table b. Marth sold the armchair for 20 percent more than she sold the coffee table AC=E

2. Warehouse W's revenue from the sale of sofas was what percent greater this year than it was last year?

a. Warehouse W sold 10 percent more sofas this year than it did last year b. Warehouse W's selling price per sofa was 30$ greater this year than it was last year. AC=E C-traps

1. Are at least 10 percent of the people in Country X who are 65 years older employed? a. In Country X, 11.3 percent of the population is 65 years old or older b. In Country X, of the population 65 years old or older, 20% of the men and 10% of the women are employed AC =B

Thanks for a great thread. Here's my contribution...

If d denotes a decimal, is d >= 0.5? i) When d is rounded off to nearest 10th, the result is 0.5 ii) When d is rounded off to nearest integer, the result is 1.

Answer is B, and if you fast enough you might be driven into a C trap.

25. If x and y are consecutive odd integers, what is the sum of x and y? (1) The product of x and y is negative. (2) One of the integers is equal to –1. Answer: A

Second condition is obviously insufficient and strongly support first one that seems to be insufficient. 7-t61637

___

What is the perimeter of quadrangle ABCD in which a circle is inscribed? 1) AB+DC=8 2) BC=5 Answer: A

Second condition is obviously insufficient and strongly support first one that seems to be insufficient. 7-t61627

___

A startting line up of a team consists of x men and y women. There are also 4 reserve players, 2 of whom are men. If one of the starting players is unable to play and needs to be replaced by one of the reserves, what is the probability that the number of women on the starting team will increase? 1) x+y =12 2) x/y=1/3 Answer: B

the problem depends on the ratio x/y rather than absolute value of x and y. This subtype of C-trap problems is a typical one for %- and average- problems. 7-t61627

___

A jewelry dealer initially offered a bracelet for sale at an asking price that would give a profit to the dealer of 40 percent of the original cost. What was the original cost of the bracelet? (1) After reducing this asking price by 10 percent, the jewelry dealer sold the bracelet at a profit of $403. (2) The jewelry dealer sold the bracelet for $1,953. Answer: A

Second condition is obviously insufficient and strongly support first one that seems to be insufficient. 7-t61415 _________________

1. First of all, I think the problem is a very tough one. 2. after I read second condition that is obviously insufficient, I thought that the problem looks like a C-trap. 3. It is obvious that for any quadrangle both conditions are insufficient. Therefore, "inscribed" restriction of the quadrangle may be crucial. 4. On the basis of my steps 2&3, I tried to find properties of the quadrangle with "inscribed" restriction and to proof that the first condition is sufficient and answer is A. _________________

Data Sufficiency (DS) Traps [#permalink]
18 Mar 2009, 16:37

Expert's post

I was digging through my files today and found a doc I wrote a long time ago about Data Sufficiency (DS) Traps - I hope this helps someone. Perhaps someone can add more examples to make this thread more complete.

Data Sufficiency Traps

1. At the first glance there is not enough information to solve the problem, but when actually attempted, the problems appears solvable.

What is the volume of a box with dimensions a, b, and c? i. a = \frac{18}{bc} ii. b = 2, c = 4

2. Both statements 1 and 2 are identical but masked.

What is the value of x?

i. x + 2y = 6 ii. 4y + 2x = 12

3. Do not assume anything on data sufficiency.

On January 1st Thomas deposited $2,000 into an interest bearing checking account. If he made no withdrawals, what was the total amount Thomas had in the checking account on December 31st of the same year?

i. Thomas deposited an additional $4,000 throughout the year ii. The checking account earned 7 percent simple interest You cannot assume that the deposits were uniform or in equal installments

How many kiloliters of water are in a reservoir?

i. If the reservoir were filled to capacity, there would be 430 more kiloliters in the reservoir. ii. The reservoir is normally 65 percent full. We don't know if these are "normal" conditions. Insufficient.

4. What ETS often does on harder DS questions, it gives the first piece of info as insufficient, and then, naturally, you move on to the second and then when you see that it is sufficient, you conclude that both are enough – C. Be careful to evaluate both statements one by one. If it is hard for you, try starting from the second statement instead of the first.

5. When you solve a medium/hard DS question, play a game with ETS, find the answer to the puzzle. If you looked through the both pieces of info and it seems both are sufficient, try proving that one is irrelevant or if both pieces are needed, try to prove that only one will be enough; try challenging the question author; give him a hard time. It will often pay off. 6. Watch out for Yes/No data sufficiency questions. 7. Make an analysis of your mistakes and see what DS questions cause the most problems. 8. Make sure you don’t confuse D and C answer choices _________________

Re: Data Sufficiency (DS) Traps [#permalink]
05 Feb 2010, 11:26

I got the spy girl question mentioned at the top no later than this afternoon on a gmat prep test. I answered E but the correct answer given is C. Can someone help please ?

What fraction of this year's graduating students at a certain college are males? a. Of this year's graduating students, 33 percent of the males and 20 percent of the females transferred from another college. b. Of this year's graduating students, 25 percent transferred from another college.

Re: Data Sufficiency (DS) Traps [#permalink]
05 Feb 2010, 15:56

arnaudl wrote:

I got the spy girl question mentioned at the top no later than this afternoon on a gmat prep test. I answered E but the correct answer given is C. Can someone help please ?

What fraction of this year's graduating students at a certain college are males? a. Of this year's graduating students, 33 percent of the males and 20 percent of the females transferred from another college. b. Of this year's graduating students, 25 percent transferred from another college.

From S2, if we have m males and f females, (m+f)/4 transfered from another college. From S1, m/3 + f/5 transferred from another college. These are equal:

so the ratio of men to women is 3 to 5, and 3/8 of all students are male.

Alternatively you could recognize that the two statements combined give us a weighted average situation; from the first statement we know the % from each group who transferred, and from the second statement we know the % of the two groups combined who transferred, from which it is always possible to find the ratio of the two groups; there's no need to calculate anything here.

I'm not sure why the answer is indicated as E in the post at the beginning of this thread. _________________

Nov 2011: After years of development, I am now making my advanced Quant books and high-level problem sets available for sale. Contact me at ianstewartgmat at gmail.com for details.