CAT Quantitative Aptitude Questions | CAT Number Systems - HCF and LCM questions

CAT Questions | Number Theory | HCF basics

The question is about HCF basics. We need to find out the possible pairs of (x,y), which satisfies the given conditions about HCF and their sum. In CAT Exam, one can generally expect to get 1~2 questions from CAT Number Systems: HCF and LCM. In multiple places, extension of HCF and LCM concepts are tested by CAT Exam and one needs to understand HCF and LCM to be able to answer the same. CAT Number theory is an important topic with lots of weightage in the CAT Exam. Make use of 2IIMs Free CAT Questions, provided with detailed solutions and Video explanations to obtain a wonderful CAT score.

Question 4: How many pairs of positive integers x, y exist such that HCF of x, y = 35 and sum of x and y = 1085?

  1. 12
  2. 8
  3. 15
  4. 30

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Explanatory Answer

Method of solving this CAT Question from Number Theory - HCF and LCM: If we " take away" the HCF from two numbers, the pair of numbers we are left with have to be coprime. This is a fabulous starting point for a number of questions.

Let HCF of (x, y) be h. Then we can write x = h * a and y = h * b. Furthermore, note that HCF (a, b) = 1. This is a very important property. One that seems obvious when it is mentioned but a property a number of people overlook.

So, we can write x = 35a; y = 35b

x + y = 1085 => 35(a + b) = 1085. => (a + b) = 31. We need to find pairs of co-prime integers that add up to 31. (Another way of looking at it is to find out integers less than 31 those are co-prime with it or phi(31) as had mentioned. More on this wonderful function in another post).

Since 31 is prime. All pairs of integers that add up to 31 will be co-prime to each other. Or, there are totally 15 pairs that satisfy this condition.

The question is "How many pairs of positive integers x, y exist such that HCF of x, y = 35 and sum of x and y = 1085?"

Hence the answer is "15"

Choice C is the correct answer.


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