GreenlightTestPrep wrote:
If x is an even positive integer that is NOT a factor of 30!, what is the smallest possible value of x?
A) 58
B) 62
C) 66
D) 68
E) 72
Take a moment to learn the following property; it will provide a useful way to think of divisibility questions.For questions involving divisibility, divisors, factors and multiples, we can say:If N is divisible by k, then k is "hiding" within the prime factorization of NConsider these examples:
24 is divisible by
3 because 24 = (2)(2)(2)
(3)Likewise, 70 is divisible by
5 because 70 = (2)
(5)(7)
And 112 is divisible by
8 because 112 = (2)
(2)(2)(2)(7)
And 630 is divisible by
15 because 630 = (2)(3)
(3)(5)(7)
-----ONTO THE QUESTION!---------------------
30! = (30)(29)(28)(27).......(4)(3)(2)(1)Let's check each answer choice....
A) 58
58 =
(29)(2)Since 30! = (30)(
29)(28)(27).......(4)(3)(
2)(1), we can conclude that 30! is divisible by 58
Eliminate answer choice A
B) 62
62 =
(2)(31)Since there are no
31's hiding in the prime factorization of 30!, we can conclude that 30! is NOT divisible by 62
Answer: B
For "fun," let's show that 30! is not divisible by each of the remaining three answer choices...
C) 66
66 =
(2)(3)(11)Since 30! = (30)(29)(28)(27)....... (
11)(10)(9)...(4)(
3)(
2)(1), we can conclude that 30! is divisible by 66
Eliminate answer choice C
D) 68
68 =
(2)(2)(17)Since 30! = (30)(29)(28)(27)....... (
17)...
(2)(2)(3)(2)(1), we can conclude that 30! is divisible by 68
Eliminate answer choice D
E) 72 =
(18)(2)(2)Since 30! = (30)(29)(28)(27)....... (
18)...
(2)(2)(3)(2)(1), we can conclude that 30! is divisible by 72
Eliminate answer choice E