Five positive integers less than 100 have exactly 12 factors (inclusive of 1 and itself).
What is the sum of these five numbers?
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Could you elaborate a little more how you found the numbers, just by guessing and trying out? Also I think equal signs shouldn't be used like that as 9 6 = 1 2 .
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If n=p1^a p2^b p3^c... Then it'll have (a+1)(b+1)(c+1)... Factors! Now every number can expressed as a product of all the primes to some powers: n=2^a 3^b 5^c... 12 can be 2 2 3, 2 3 2, 3 2 2, 4 3, 3 4, 6 2 or 2 6. Now you can substract one to all of these factors and plot them on n=2^a 3^b 5^c... And find the 5 integers! It won't take too much time because the numbers get greater than 100 very rapidly. Hope you understood :)
See comment above by Romain.
In order to find the 5 numbers, I first decided on a set of the first 6 factors, then I tried to find a number that has those factors. Each of those 6 factors would need to multiply by another unique factor to equal the number. So a number n with factors
a , b , c , d , e , f , u , v , w , x , y , z
would be equal to
a × z = b × y = c × x = d × w = e × v = f × u = n
In order for this to work, the square root of the number must be greater than the first 6 factors and less than the last 6 factors.
So, I first started with 1 , 2 , 3 , 4 , 5 , 6 as the first 6 factors. The LCM of those numbers happens to be 6 0 . So does 6 0 work?
Factors of 6 0 = ( 1 , 2 , 3 , 4 , 5 , 6 , 1 0 , 1 2 , 1 5 , 2 0 , 3 0 , 6 0 )
Yes it does. However this didn't always work. For example, the LCM of 1 , 2 , 3 , 4 , 6 , 8 is only 2 4 .
Factors of 2 4 = ( 1 , 2 , 3 , 4 , 6 , 8 , 1 2 , 2 4 )
Only 8 factors. However, if you multiply 2 4 by an existing factor, like 3 or 4 , you can get a number that works.
2 4 × 3 = 7 2
Factors of 7 2 = ( 1 , 2 , 3 , 4 , 6 , 8 , 9 , 1 2 , 1 8 , 2 4 , 3 6 , 7 2 )
2 4 × 4 = 9 6
Factors of 9 6 = ( 1 , 2 , 3 , 4 , 6 , 8 , 1 2 , 1 6 , 2 4 , 3 2 , 4 8 , 9 6 )
Both 7 2 and 9 6 have 12 factors.
So you can continue this process to find the other 2 numbers. One thing that is useful to note is that 1 , 2 , 3 , 4 will always be the first 4 factors, because if they aren't, you will not be able to find 6 factors less than 10, which means the resulting number will be greater than 100, or will not have 12 factors.
L C M ( 1 , 2 , 3 , 4 , 6 , 7 ) = 8 4
Factors of 8 4 = ( 1 , 2 , 3 , 4 , 6 , 7 , 1 2 , 1 4 , 2 1 , 2 8 , 4 2 , 8 4 )
L C M ( 1 , 2 , 3 , 5 , 6 , 9 ) = 9 0
Factors of 9 0 = ( 1 , 2 , 3 , 5 , 6 , 9 , 1 0 , 1 5 , 1 8 , 3 0 , 4 5 , 9 0 )
Both of those numbers also have 12 factors. So since we have found the 5 factors, all we need to do is add them.
6 0 + 7 2 + 8 4 + 9 0 + 9 6 = 4 0 2
12=2 × 3 = 4 × 3 =2 × 2 × 3
let N= 2 p 3 q 5 r 7 s ⋯
so from factors of 12 we can get p,q r,s and also can get five required integers
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9 6 = 2 5 ∗ 3 1 6x2 factors
9 0 = 2 1 ∗ 3 2 ∗ 5 1 2x3x2 factors
8 4 = 2 2 ∗ 3 1 ∗ 7 1 3x2x2 factors
7 2 = 2 3 ∗ 3 2 4x3 factors
6 0 = 2 2 ∗ 3 1 ∗ 5 1 3x2x2 factors
9 6 + 9 0 + 8 4 + 7 2 + 6 0 = 4 0 2