Beat The GMAT Team's Featured Posts for 2010 - Data Sufficiency

by Beat The GMAT, Jan 14, 2011

2010 has been a special year for the Beat The GMAT community. This fall we celebrated our 100,000^th member and this important milestone reminded us all that we are, first and foremost, a community built by our members for our members. We wanted to thank all of our members, both regular test takers and experts, by featuring some of our favorite threads from our forums. Your wonderful contributions are much appreciated by MBA hopefuls everywhere!

Data Sufficiency is a question type youll probably only see on the GMAT. It tests your knowledge of quant by asking you to evaluate two pieces of information and decide whether they are sufficient to answer a question. Because most test takers havent seen DS questions before studying for the GMAT, we often hear complaints about the difficulty of tackling these strange problems. Were here to help! You can check out an awesome video series put together by Brent Hanneson, read about DS in our library of articles or post your questions in the DS forum.

Absolute values and inequalities quite the mix

iLdern posted a DS question involving both absolute values and inequalities. What we liked about this thread was that one question generated three different solutions, by iLdern, Jim@Grockit and duongthang. Should you break it down using cases, apply number properties or pick numbers? Its up to you to decide. Heres a part of that conversation:

Is |x+y|>|x-y|? If one or both variables equals 0, then no (the expressions are equal); likewise, if the signs of x and y are different, then no, |x+y|<|x-y|. If the signs of x and y are the same, then yes, |x+y|>|x-y|.

Read the rest of the thread here.

Terminating decimals

We work with decimals all the time on the GMAT, but chrisjim5s question is a bit special. It really challenges you to dig deeper into the matter of divisibility! Ian Stewart and fskilnik both post useful comments on the matter, with no shortage of examples:

If the denominator has any prime factor besides 2 or 5, the fraction will give a *repeating* (infinite) decimal. If the only prime factors of the denominator are 2 and/or 5, the fraction will give a terminating decimal.

So fractions like 3/16 (only prime factor of denominator is 2), 9/125 (only prime factor of denominator is 5) and 3/40 (only prime factors of denominator are 2 and 5) will all produce terminating decimals.

You can find the full discussion here.

Be careful with inequalities

One of the simplest rules you need to remember when dealing with inequalities is potentially the rule that people forget about most often: if youll multiply both sides by something, make sure that something is positive or else you need to flip the sign! This is particularly important in DS, where you dont have any answer choices to plug in in case youre lost. achieve_dream posted a question involving this recently. Geva@MasterGMAT then replied with the following:

The problem is that you can't multiply an inequality by a unknown without knowing the unknown's sign: If sp+q is positive, then you can multiply without any changes, BUT if 2p+q is negative, you would need to flip the inequality sign because of multiplying by a negative number. Thus, without any indication of whether wp+q is positive or negative, you can't multiply because you don't know what to do with the sign.

You can check out the full forum post here.

Of course, our forums are just teeming with awesome advice. If you happen to stumble upon a gem, please share it with us in the comments section!