Is the positive integer n divisible by 18?
1.) n^2 is divisible by 18
2.) 2n is divisible by 18
OA is C
please explain!!
Number Properties- divisibility
This topic has expert replies
-
- Master | Next Rank: 500 Posts
- Posts: 146
- Joined: Wed Aug 27, 2008 5:41 am
- Thanked: 3 times
my answer is C
statement 1: insufficient...
if n=6 , then n^2 = 36 which is divisible by 18 but n=6 is not divisible by 18
but if n=18 , then n^2 and n both are divisible by 18
giving both yes or no as a ans. so statement 1 is insufficient
statement 2 : insufficient
according to given condition
2n/18= x ( where x is an integer )
or n=x*18/12=x*9 so n can be 9,18,27,36,........
this situation also give both yes and no as answer ... so it is also insufficient
taking both statement together .... the answer is always yes (take few numbers which statisfy both the conditions )
so both the statement together are sufficient
statement 1: insufficient...
if n=6 , then n^2 = 36 which is divisible by 18 but n=6 is not divisible by 18
but if n=18 , then n^2 and n both are divisible by 18
giving both yes or no as a ans. so statement 1 is insufficient
statement 2 : insufficient
according to given condition
2n/18= x ( where x is an integer )
or n=x*18/12=x*9 so n can be 9,18,27,36,........
this situation also give both yes and no as answer ... so it is also insufficient
taking both statement together .... the answer is always yes (take few numbers which statisfy both the conditions )
so both the statement together are sufficient
- Stuart@KaplanGMAT
- GMAT Instructor
- Posts: 3225
- Joined: Tue Jan 08, 2008 2:40 pm
- Location: Toronto
- Thanked: 1710 times
- Followed by:614 members
- GMAT Score:800
From the stem, we know that n is a positive integer. Our task: to determine if n is divisible by 18 or, in other words, if n has, among its prime factors, 2, 3 and 3.apoorva.srivastva wrote:Is the positive integer n divisible by 18?
1.) n^2 is divisible by 18
2.) 2n is divisible by 18
1) n^2 is divisible by 18.
So n^2 has factors of 2, 3 and 3. Does this mean that n also has those factors? No: we know that n^2 will have pairs of primes, so n^2 has to include at least one more "2", but that doesn't mean that n has to have two "3"s.
For example, n^2 could be 2*2*3*3 = 36, which means that n=6, which isn't a multiple of 18.
Or, n^2 could be 2*2*2*2*3*3*3*3 = 36^2, which means that n=36, which is a multiple of 18.
Insufficient, eliminate A and D.
2) 2n is divisible by 18.
Well, if 2n is divisible by 18, then n must be divisible by 18/2 = 9.
We could pick n=9 to get a "no" answer or n=18 to get a "yes" answer.
Insufficient, eliminate B.
Since neither statement was sufficient alone, we need to combine.
From (1), we know that n must be even.
From (2), we know that n is a multiple of 9.
So, together, we know that n is an even multiple of 9.
Is every even multiple of 9 divisible by 18? YES - sufficient, choose C.
Stuart Kovinsky | Kaplan GMAT Faculty | Toronto
Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course
- eaakbari
- Master | Next Rank: 500 Posts
- Posts: 435
- Joined: Mon Mar 15, 2010 6:15 am
- Thanked: 32 times
- Followed by:1 members
Is the positive integer n divisible by 18?
1.) n^2 is divisible by 18
2.) 2n is divisible by 18
Stem:
n>0
we need to know whether n is div by 18 or n has factors 3^2 and 2
Statement one
n^2 is div by 18.
or n^2 is div by 2 and 3^2.
Hence any number having factors 2 and 3 will satisfy like 6 and 6 is clearly not div by 18. Hence Insuff
Statement two
2n is div by 2 *3^2.
That implies n contains factors 3^2 , But we dont know about 2. Hence Insuff
Combining
Statement one told us that n has factors 2 and 3
Statement 2 told us it has factor 3^2
We need factors 2 *3^2
which we clearly have
Hence suff
Hence C