Numbers having 12 factors

Five positive integers less than 100 have exactly 12 factors (inclusive of 1 and itself).

What is the sum of these five numbers?


The answer is 402.

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3 solutions

Paul Fournier
Jun 22, 2016

96 = 2 5 3 1 96=2^5*3^1 6x2 factors

90 = 2 1 3 2 5 1 90=2^1*3^2*5^1 2x3x2 factors

84 = 2 2 3 1 7 1 84=2^2*3^1*7^1 3x2x2 factors

72 = 2 3 3 2 72=2^3*3^2 4x3 factors

60 = 2 2 3 1 5 1 60=2^2*3^1*5^1 3x2x2 factors

96 + 90 + 84 + 72 + 60 = 402 96+90+84+72+60=402

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 96 12 96 \neq 12 .

Kai Ott - 4 years, 11 months ago

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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 :)

Romain Farthoat - 4 years, 11 months ago

See comment above by Romain.

Paul Fournier - 4 years, 11 months ago
D C
Jun 23, 2016

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 n with factors

a , b , c , d , e , f , u , v , w , x , y , z 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 a \times z = b \times y = c \times x = d \times w = e \times v = f \times 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 1, 2, 3, 4, 5, 6 as the first 6 factors. The LCM of those numbers happens to be 60 60 . So does 60 60 work?

Factors of 60 = ( 1 , 2 , 3 , 4 , 5 , 6 , 10 , 12 , 15 , 20 , 30 , 60 ) 60 = (1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60)

Yes it does. However this didn't always work. For example, the LCM of 1 , 2 , 3 , 4 , 6 , 8 1, 2, 3, 4, 6, 8 is only 24 24 .

Factors of 24 = ( 1 , 2 , 3 , 4 , 6 , 8 , 12 , 24 ) 24 = (1, 2, 3, 4, 6, 8, 12, 24)

Only 8 factors. However, if you multiply 24 24 by an existing factor, like 3 3 or 4 4 , you can get a number that works.

24 × 3 = 72 24 \times 3 = 72

Factors of 72 = ( 1 , 2 , 3 , 4 , 6 , 8 , 9 , 12 , 18 , 24 , 36 , 72 ) 72 = (1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72)

24 × 4 = 96 24 \times 4 = 96

Factors of 96 = ( 1 , 2 , 3 , 4 , 6 , 8 , 12 , 16 , 24 , 32 , 48 , 96 ) 96 = (1, 2, 3, 4, 6, 8, 12, 16, 24, 32, 48, 96)

Both 72 72 and 96 96 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 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 ) = 84 LCM(1, 2, 3, 4, 6, 7) = 84

Factors of 84 = ( 1 , 2 , 3 , 4 , 6 , 7 , 12 , 14 , 21 , 28 , 42 , 84 ) 84 = (1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42, 84)

L C M ( 1 , 2 , 3 , 5 , 6 , 9 ) = 90 LCM(1, 2, 3, 5, 6, 9) = 90

Factors of 90 = ( 1 , 2 , 3 , 5 , 6 , 9 , 10 , 15 , 18 , 30 , 45 , 90 ) 90 = (1, 2, 3, 5, 6, 9, 10, 15, 18, 30, 45, 90)

Both of those numbers also have 12 factors. So since we have found the 5 factors, all we need to do is add them.

60 + 72 + 84 + 90 + 96 = 402 60 + 72 + 84 + 90 + 96 = 402

Kushal Bose
Jun 23, 2016

12=2 × \times 3 = 4 × \times 3 =2 × \times 2 × \times 3

let N= 2 p 3 q 5 r 7 s 2^{p}3^{q}5^{r}7^{s}\cdots

so from factors of 12 we can get p,q r,s and also can get five required integers

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