CAT Number Theory - Interesting question

This is an interesting question from Number Theory. Slightly unconventional, but interesting nevertheless.

Question
The sum of the factors of a number is 124. What is the number?

Correct Answer
Number could be 48 or 75

Explanatory Answer
This video gives the solution for this question. Given below the video is the explanation (in words)



Any number of the form paqbrc will have (a+1) (b+1)(c+1) factors, where p, q, r are prime. (This is a very important idea)
For any number N of the form paqbrc, the sum of the factors will be (1 + p1 + p2 + p3+ …+ pa) (1 + q1 + q2 + q3+ …+ qb) (1 + r1 + r2 + r3+ …+ rc).
Sum of factors of number N is 124. 124 can be factorized as 22 * 31. It can be written as 4 * 31, or 2 * 62 or 1 * 124.
2 cannot be written as (1 + p1 + p2 …pa) for any value of p.
4 can be written as (1 + 3)
So, we need to see if 31 can be written in that form.
The interesting bit here is that 31 can be written in two different ways
31 = (1 + 21 + 22+ 23 + 24)
31 = ( 1 + 5 + 52)
Or, the number N can be 3 * 24 or 3 * 52. Or N can be 48 or 75.

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IIM CAT Preparation Tips: CAT Number Theory - Interesting question

Dec 3, 2012

CAT Number Theory - Interesting question

This is an interesting question from Number Theory. Slightly unconventional, but interesting nevertheless.

Question
The sum of the factors of a number is 124. What is the number?

Correct Answer
Number could be 48 or 75

Explanatory Answer
This video gives the solution for this question. Given below the video is the explanation (in words)



Any number of the form paqbrc will have (a+1) (b+1)(c+1) factors, where p, q, r are prime. (This is a very important idea)
For any number N of the form paqbrc, the sum of the factors will be (1 + p1 + p2 + p3+ …+ pa) (1 + q1 + q2 + q3+ …+ qb) (1 + r1 + r2 + r3+ …+ rc).
Sum of factors of number N is 124. 124 can be factorized as 22 * 31. It can be written as 4 * 31, or 2 * 62 or 1 * 124.
2 cannot be written as (1 + p1 + p2 …pa) for any value of p.
4 can be written as (1 + 3)
So, we need to see if 31 can be written in that form.
The interesting bit here is that 31 can be written in two different ways
31 = (1 + 21 + 22+ 23 + 24)
31 = ( 1 + 5 + 52)
Or, the number N can be 3 * 24 or 3 * 52. Or N can be 48 or 75.

Labels: , , , ,

3 Comments:

At 5:23 PM , Blogger Bharathwaj.U said...

This comment has been removed by the author.

 
At 5:25 PM , Blogger Bharathwaj.U said...

sum of factors of 48 is 124

 
At 1:41 PM , Blogger ayan biswas said...

Finding 31 can be written as
31= (1 + 21 + 22+ 23 + 24)
31 = ( 1 + 5 + 52), is difficult.Is there a trick to find this?

 

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