Some LCM/GCF Properties | Number Properties

Sometimes students come up with concepts they read in books for which they are looking for explanations. Actually, in Quant, you can establish innumerable inferences from the theory of any topic. The point is that you should be comfortable with the theory. You should be able to deduce your own inferences from your understanding of the topic. If you come across some so-called rules, you should be able to say why they hold. Let’s discuss a couple of such rules from number properties regarding GCF and LCM. Many of you might be reading them for the first time. Stop and think why they must hold.

Rule 1: Consecutive multiples of ‘ $x$ ‘ have a GCF of ‘ $x$ ‘.

Explanation: What do we mean by consecutive multiples of $x$ ? They are the consecutive terms in the multiplication table of $x$ .

For example, $4x$ and $5x$ are consecutive multiples of $x$ . So are $18x$ and $19x$ …

What will be the greatest common factor of $18x$ and $19x$ ? We know that $x$ is their common factor. Do 18 and 19 have any common factors (except 1)? No. So greatest common factor will be $x$ . Take any two consecutive numbers. They will have no common factors except 1. Hence, if we have two consecutive factors of $x$ , their GCF will always be $x$ .

Can you derive some of your own ‘rules’ based on this now?

Let’s give you some ideas:

Two consecutive integers have GCF 1.

Two consecutive odd multiples of $x$ have GCF $x$ .

Rule 2: The G.C.F of two distinct numbers cannot be larger than the difference between the two numbers.

Explanation: GCF is a factor of both the numbers. Say, the GCF of two distinct numbers is $x$ . This means the two numbers are $mx$ and $nx$ where $m$ and $n$ have no common factor. What can be the smallest difference between $m$ and $n$ ? $m$ and $n$ cannot be equal since the numbers are distinct. The smallest difference between them can be 1 i.e. they can be consecutive numbers. In that case, the difference between $mx$ and $nx$ will be $x$ which is equal to the GCF. If $m$ and $n$ are not consecutive integers, the difference between them will be much larger than $x$ . The difference between $mx$ and $nx$ cannot be less than $x$ .

Say, GCF of two numbers is 6. The numbers can be 6 and 12(GCF = 6) or 6 and 30(GCF = 6) etc but they cannot be 6 and 8 since both numbers must have 6 as a factor.

Let’s look at a question based on these concepts now.

Question 1: What is the greatest common factor of ${x}$ and ${y}$ ?

Statement 1: ${x}$ and ${y}$ are both divisible by 4.

Statement 2: ${x - y = 4}$

Solution:

Statement 1: $x$ and $y$ are both divisible by 4

We know that 4 is a factor of both $x$ and $y$ . But is it the highest common factor? We do not know. There could be another factor common between $x$ and $y$ and hence highest common factor could be greater than 4. e.g. 4 and 16 have 4 as the highest common factor but 12 and 36 have 12 as the highest common factor though both pairs have 4 as a common factor.

Statement 2: $x - y = 4$

We know that $x$ and $y$ differ by 4. So their GCF cannot be greater than 4 (as discussed above). The GCF could be any of 1/2/4 e.g. 7 and 11 have GCF of 1 while 2 and 6 have GCF of 2.

Taking both statements together: From statement 1, we know that $x$ and $y$ have 4 as a common factor. From statement 2, we know that $x$ and $y$ have one of 1/2/4 as highest common factor. Hence 4 is the highest common factor.

Answer (C).

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