The single most-tested topic on the Quantitative Section is something called "Number Properties." In a way, this statement, while absolutely true, is a bit deceptive. It’s like saying the biggest category at a particular zoo is mammals: even if that is completely true, that statement alone doesn’t give us a particular clear idea of what kinds of critters we are likely to encounter at the zoo. Much in the same way, "number properties" is a vast category, just like "mammals", and simply knowing that you will have to know "number properties" doesn’t necessarily narrow down specifically what you will have to know. This post is designed to give an overview of this vast topic.
Let’s first talk about the constituents of this realm. When I say the word "numbers", what do you think? If the only thing that comes to mind is (
Numbers live on what grade-school teachers call the "number line" and what mathematicians call the "real number line", a continuous infinity of values. (The more you think about the idea of continuous infinity, the more it boggles the mind!) The real-number line is a real of perfect fairness — the number -137/8 is just as much a legitimate member of the number line as is 4. If you have any prejudice that, for example (
It’s particular important if you are choosing numbers plugging in, say to test algebraic expression in a Data Sufficiency question, to remember to test numbers of all categories.
In addition to knowing the infinite implications concerning the word "number", two further terms you should know are "integers" (...
The "parts" between the "wholes" can be represented in two ways: fractions and decimals. Both can be either positive or negative. Mathematicians have both systems for these "partial" numbers because each system has its own advantages under certain circumstance. Technically, "fractions" and "decimals" don’t cover exactly the same turf, because, as I discuss in that post on decimals, fractions include only the rational numbers, whereas decimals include both rationals and irrational numbers, an infinitely bigger set. Thus, even though there’s an infinity of fractions between
The positive integers (
One of the hardest things the QUANT will ask you to do with positive integers is to count. This may sound paradoxical, as counting is one mathematical thing even the most inveterate math-phobe can do, but here, by "counting", I mean problems like: "There are three boys in blue shirts, five boys in green shirts, and four girls in blue dresses. How many ways can they sit in seven seats if blah blah blah?" Those arrangements and sets and combinations — that’s what the QUANT will have you count. Also, remember the trick of inclusive counting, which the QUANT loves to test.
Number properties is a huge topic: if you follow all those links, you will have a good understanding of the lay of the land. Finally, here’s a very easy practice question, to get you warmed up to thinking about this topic as it appear in test-format questions:
Q1. The Greatest Common Factor (GCF) of 48 and 72 is
Find the prime factorizations of both numbers, and mark the factors they have in common:
48 = 2*2*2*2*3
72 = 2*2*2*3*3
They both have at least three factors of 2 and one factor of 3, and 2*2*2*3 = 24, so this is the GCF.
Answer = D.