GMAT Math: Terminating and Repeating Decimals

By - Apr 25, 09:00 AM Comments [0]

The topic of decimals, and patterns of decimals, seems to be of slightly greater interest to GMAC in the GMAT OG13e than in previous editions.  What decimals terminate?  What decimals repeat?  In this post, we’ll take a look at these questions.

 

Rational Numbers

Integers are positive and negative whole numbers, including zero.  Here are the integers:

{ … -3, -2, -1, 0, 1, 2, 3, …}

When we take a ratio of two integers, we get a rational number.  A rational number is any number of the form a/b, where a & b are integers, and b ≠ 0. Rational numbers are the set of all fractions made with integer ingredients.   Notice that all integers are included in the set of rational numbers, because, for example, 3/1 = 3.

 

Rational Numbers as Decimals

When we make a decimal out of a fraction, one of two things happens.  It either terminates (comes to an end) or repeats (goes on forever in a pattern).  Terminating rational numbers include:

1/2 = 0.5

1/8 = 0.125

3/20 = 0.15

9/160 = 0.5625

 

Repeating rational numbers include:

1/3 = 0.333333333333333333333333333333333333…

1/7 = 0.142857142857142857142857142857142857…

1/11 = 0.090909090909090909090909090909090909…

1/15 = 0.066666666666666666666666666666666666…

 

When Do Rational Number Terminate?

The GMAT won’t give you a complicated fraction like 9/160 and expect you to figure out what its decimal expression is.  BUT, the GMAT could give you a fraction like 9/160 and ask whether it terminates or not.  How do you know?

Well, first of all, any terminating decimal (like 0.0376) is, essentially, a fraction with a power of ten in the dominator; for example, 0.0376 = 376/10000 = 47/1250.  Notice we simplified this fraction, by cancelling a factor of 8 in the numerator.  Ten has factors of 2 and 5, so any power of ten will have powers of 2 and powers of 5, and some might be canceled by factors in the numerator , but no other factors will be introduced into the denominator.  Thus, if the prime factorization of the denominator of a fraction has only factors of 2 and factors of 5, then it can be written as something over a power of ten, which means its decimal expression will terminate.

If the prime factorization of the denominator of a fraction has only factors of 2 and factors of 5, the decimal expression terminates.  If there is any prime factor in the denominator other than 2 or 5, then the decimal expression repeats.  Thus,

 

1/24 repeats (there’s a factor of 3)

1/25 terminates (just powers of 5)

1/28 repeats (there’s a factor of 7)

1/32 terminates (just powers of 2)

1/40 terminates (just powers of 2 and 5)

 

Notice, as long as the fraction is in lowest terms, the numerator doesn’t matter at all. Since 1/40 terminates, then 7/40, 13/40, or any other integer over 40 also terminates.Since 1/28 repeats, then 5/28 and 15/28 and 25/28 all repeat; notice, though that 7/28 doesn’t repeat, because of the cancellation: 7/28 = 1/4 = 0.25.

 

Shortcut Decimals:

There are certain decimals that are good to know as shortcut, both for fraction-to-decimal conversions and for fraction-to-percent conversions.  These are

 

1/2 = 0.5

1/3 = 0.33333333333333333333333333…

2/3 = 0.66666666666666666666666666…

1/4 = 0.25

3/4 = 0.75

1/5 = 0.2 (and times 2, 3, and 4 for other easy decimals)

1/6 = 0.166666666666666666666666666….

5/6 = 0.833333333333333333333333333…

1/8 = 0.125

1/9 = 0.111111111111111111111111111… (and times other digits for other easy decimals)

1/11 = 0.09090909090909090909090909… (and times other digits for other easy decimals)

 

Irrationals

There’s another category of decimals that don’t terminate (they go on forever) and they have no repeating pattern.   These numbers, the non-terminating non-repeating decimals, are called the irrational numbers.  It is impossible to write any one of them as a ratio of two integers.  Mr. Pythagoras (c. 570 – c. 495 bce) was the first to prove a number irrational: he proved that the square-root of 2 — [pmath]sqrt(2)[/pmath]  — is irrational.  We now know: all square-roots of integers that don’t come out evenly are irrational.  Another famous irrational number is [pmath]pi[/pmath], or pi, the ratio of a circle’s circumference to its diameter.  For example,

[pmath]pi[/pmath] = 3.1415926535897932384626433832795028841971693993751058209749445923078164

0628620899862803482534211706798214808651328230664709384460955058223172535940812848111745

0284102701938521105559644622948954930381964428810975665933446128475648233786783165271201

909145648566923460348610454326648213393072602491412737…

 

That’s the first three hundred digits of pi, and the digits never repeat: they go on forever with no repeating pattern.  There are infinite many irrational numbers: in fact, the infinity of irrational numbers is infinitely bigger than the infinity of the rational numbers, but that gets into some math (http://en.wikipedia.org/wiki/Aleph_number) that is much more advanced than the GMAT.

 

Practice Question

1) [pmath]{0.16666…/0.44444…} = [/pmath]

(A) 2/27

(B) 3/2

(C) 3/4

(D) 3/8

(E) 9/16

 

Practice Question Explanation

1) From our shortcuts, we know 0.166666666666… = 1/6, and 0.444444444444… = 1/9.  Therefore (1/6)*(9/4) = 3/8.  Answer = D

 

This post was written by Mike McGarry, GMAT Expert at Magoosh, and originally posted here.

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