# CAT Practice : Number System - HCF, LCM

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Given a property of HCF and LCM of two numbers, how do we get to the underlying numbers.

## HCF LCM

Q.5: How many pairs of positive integers x, y exist such that HCF (x, y) + LCM (x, y) = 91?
1. 10
2. 8
3. 6
4. 7

Choice B. 8

## Detailed Solution

Let us x = h * a; y = h * b
a and b are co-prime. So, LCM of (x, y) = h * a * b

So, in essence h + h * a * b = 91. Or h(ab + 1) = 91
Now, 91 can be written as 1 * 91 or 7 * 13
Or, we can have HCF as 1, LCM as 90 - There are 4 pairs of numbers like this (2, 45), (9, 10), (1, 90) and (5, 18)

We can have HCF as 7, ab + 1 as 13 => ab = 12 => 1 * 12 or 4 * 3

Or, the pairs of numbers are (7, 84) or (21, 28)

The third option is when HCF = 13, ab + 1 = 7 => ab = 6
Or (a, b) can be either (1, 6) or (2, 3)
The pairs possible are (13, 78) and (26, 39)
There are totally 8 options possible - (2, 45), (9, 10), (1, 90), (5, 18), (7, 84), (21, 28), (13, 78) and (26, 39).
8 Pairs. Choice (B).

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## More questions from Number Theory - HCF, LCM

Number Theory is one of the most heavily tested topics. Within this, one should get the basics on factors, multiples, HCF, LCM very clear before moving on to the tougher sets.