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Re: If X = the product of four distinct prime numbers, how many [#permalink]
salsal wrote:
If X = the product of four distinct prime numbers, how many factors does X have besides 1 and itself?

(a) 10
(b) 11
(c) 12
(d) 13
(e) 14


My approach:

Let's take the four primes: 2, 3, 5 and 7 --> each of the 4 primes will be a factor of the product

All the possible combinations of the four primes will be factors (10 possible combinations):
2x3
2x5
2x7
3x5
3x7
5x7
2x3x5
2x3x7
2x5x7
3x5x7

Thus, in total there are 10 + 4 = 14 factors.

Is my reasoning correct? Is there a faster way to figure out the number of possible combinations than listing them?


I took the combinations approach as well. Instead of listing them out you I picked combinations of 3, 2 and 1. Combination of all 4 numbers is out because we are told that number and itself should be excluded.

4C3 + 4C2 + 4C1
4!/(3!1!) + 4!/(2!2!) + 4!/(3!1!)
4 + 6 + 4 = 14
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Re: If X = the product of four distinct prime numbers, how many [#permalink]
Expert Reply
salsal wrote:
If X = the product of four distinct prime numbers, how many factors does X have besides 1 and itself?

(a) 10
(b) 11
(c) 12
(d) 13
(e) 14


My approach:

Let's take the four primes: 2, 3, 5 and 7 --> each of the 4 primes will be a factor of the product

All the possible combinations of the four primes will be factors (10 possible combinations):
2x3
2x5
2x7
3x5
3x7
5x7
2x3x5
2x3x7
2x5x7
3x5x7

Thus, in total there are 10 + 4 = 14 factors.

Is my reasoning correct? Is there a faster way to figure out the number of possible combinations than listing them?


Let the numbers be 2,3,5,7.
Single factors are 4C1 = 4
Product of 2 numbers are also factors = 4C2 = 6
Product of 3 numbers are also factors = 4C3 = 4

Hence 14 other factors besides 1 and the number itself.

Hence option E.

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Re: If X = the product of four distinct prime numbers, how many [#permalink]
If we use 2*3*5*7 = 210

210 = 2*5*3*7

Total number of factors: (1+1) *(1+1) *(1+1) *(1+1) = 16

16 - 2 = 14, without 1 and itself. ANS E
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Re: If X = the product of four distinct prime numbers, how many [#permalink]
[quote="salsal"]If X = the product of four distinct prime numbers, how many factors does X have besides 1 and itself?

(a) 10
(b) 11
(c) 12
(d) 13
(e) 14


No. of Factors =(1+1) x (1+1) x(1+1) x(1+1)
= 16
Besides 1 & itself no. of factors X will have = 16-2 =14
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Re: If X = the product of four distinct prime numbers, how many [#permalink]
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Re: If X = the product of four distinct prime numbers, how many [#permalink]
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