Lesson: Factorial Notation

Comment on Factorial Notation

Which of the following is a multiple of 4!+6?
a)2!+3
b) 4!+12
c)5!+6
d)5!+30
e)8!+12

How do i tackle this problem Brent? Do i have to consider prime numbers?
gmat-admin's picture

Nice question!

If A and a multiple of B, then we can write A = kB for some integer k.

For example, we know that 24 is a multiple of 6, because we can write 24 = (4)(6)

4! + 6 = (4)(3)(2)(1) + 6

All MULTIPLES of 4! + 6 are in the form k(4! + 6), where k is an integers

k(4! + 6) = k[(4)(3)(2)(1) + 6]
= (k)(4)(3)(2)(1) + 6k

So, all multiples of 4! + 6 can be written in the form (k)(4)(3)(2)(1) + 6k

At this point, I SCAN the answer choices to see if any of them can be derived from the expression (k)(4)(3)(2)(1) + 6k

Notice that answer choice D, 5! + 30, can be derived if we let k = 5

That is, 5[4! + 6] = 5[(4)(3)(2)(1) + 6]
= (5)(4)(3)(2)(1) + 30
= 5! + 30

Answer: D

Did you work backwards and plugged in Answer Choice D?
gmat-admin's picture

Great question.
I have edited my response above to address that question.

Cheers.
Brent

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