What is the largest integer value of x that makes 30!/3^x an integer?
A)10
B)12
C)14
D)16
E)18
Thanks in advance!
Factorials - anyone know a really quick way
This topic has expert replies
-
- Senior | Next Rank: 100 Posts
- Posts: 62
- Joined: Thu Jul 03, 2008 4:52 am
-
- Legendary Member
- Posts: 829
- Joined: Mon Jul 07, 2008 10:09 pm
- Location: INDIA
- Thanked: 84 times
- Followed by:3 members
-
- Legendary Member
- Posts: 661
- Joined: Tue Jul 08, 2008 12:58 pm
- Location: France
- Thanked: 48 times
You need to know the number of 3 you can have in 30!
You take the multiples of 3 and you see the number of 3 you have in, it takes 20sec with a paper.
3-->1
6-->1
9-->2
...
30-->1
It gives 14
Sudhir could you develop your point cause I couldn't use it, I don't catch.
You take the multiples of 3 and you see the number of 3 you have in, it takes 20sec with a paper.
3-->1
6-->1
9-->2
...
30-->1
It gives 14
Sudhir could you develop your point cause I couldn't use it, I don't catch.
- VP_RedSoxFan
- GMAT Instructor
- Posts: 85
- Joined: Thu May 01, 2008 12:56 pm
- Location: Salt Lake City, UT
- Thanked: 24 times
- GMAT Score:750+
I'm not entirely sure how sudhir3127 got the 14, but that is what I get as well by doing the following procedure:
If I'm going to take 30! and divide by a bunch of 3's, I would like to divide by all of the 3 factors in 30! to find the largest value for x.
I can figure this out by looking at all of the factors of 30! (30,29,28, etc) that will, themselves, have factors of 3 in them. At least that will be 30, 27, 24, ... , 6, 3. I can exclude examining any of the other factors of 30! (28, 19, ect.) as dividing them by 3 will not yield an integer.
These are the factors of 30! that have at least one 3 factor in them, some will have more than one 3 as a factor as follows.
3 = 3
6 = 2*3
9 = 3*3
12 = 2*2*3
15 = 5*3
18 = 2*3*3
21 = 7*3
24 = 2*2*2*3
27 = 3*3*3
30 = 2*5*3
Now, if I count up the 3's present in my chart, I get 14. I can divide 30! by 3 fourteen times and that will cancel out all of the threes in 30! but still leave me with an integer.
Hope this helps, let me know if I need to explain some more.
If I'm going to take 30! and divide by a bunch of 3's, I would like to divide by all of the 3 factors in 30! to find the largest value for x.
I can figure this out by looking at all of the factors of 30! (30,29,28, etc) that will, themselves, have factors of 3 in them. At least that will be 30, 27, 24, ... , 6, 3. I can exclude examining any of the other factors of 30! (28, 19, ect.) as dividing them by 3 will not yield an integer.
These are the factors of 30! that have at least one 3 factor in them, some will have more than one 3 as a factor as follows.
3 = 3
6 = 2*3
9 = 3*3
12 = 2*2*3
15 = 5*3
18 = 2*3*3
21 = 7*3
24 = 2*2*2*3
27 = 3*3*3
30 = 2*5*3
Now, if I count up the 3's present in my chart, I get 14. I can divide 30! by 3 fourteen times and that will cancel out all of the threes in 30! but still leave me with an integer.
Hope this helps, let me know if I need to explain some more.
Ryan S.
| GMAT Instructor |
Elite GMAT Preparation and Admissions Consulting
www.VeritasPrep.com
Learn more about me
| GMAT Instructor |
Elite GMAT Preparation and Admissions Consulting
www.VeritasPrep.com
Learn more about me
-
- Senior | Next Rank: 100 Posts
- Posts: 62
- Joined: Thu Jul 03, 2008 4:52 am
GMAT/MBA Expert
- Ian Stewart
- GMAT Instructor
- Posts: 2621
- Joined: Mon Jun 02, 2008 3:17 am
- Location: Montreal
- Thanked: 1090 times
- Followed by:355 members
- GMAT Score:780
Nice, but I think some didn't quite understand itsudhir3127 wrote:My answer is 14.
here it goes..
30/3 + 30/3^2 + 30/3^3
10 + 3 + 1 = 14
let me know the OA,
What sudhir did:
-count how many multiples of 3 there are between 1 and 30- there are 30/3, or ten multiples of 3.
-we need to count an extra 3 for the multiples of 3^2. How many of these are there? 30/9 = 3+remainder; there will be three multiples of 3^2.
-finally, we need to count yet another 3 for the multiples of 3^3. How many of these are there? 30/27 = 1+remainder; there will be only one multiple of 3^3.
10+3+1 = 14.
The approach does depend on the fact that the range of numbers begins from 1; with a different set of numbers, the calculations would be more awkward (in a range of 30 numbers, you can have one multiple of 3^3, or you can have two multiples of 3^3).
It's an elegant approach- nicely done, sudhir.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com
ianstewartgmat.com
ianstewartgmat.com
-
- Legendary Member
- Posts: 1153
- Joined: Wed Jun 20, 2007 6:21 am
- Thanked: 146 times
- Followed by:2 members
This is sheer brilliance!!!!!!sudhir3127 wrote:My answer is 14.
here it goes..
30/3 + 30/3^2 + 30/3^3
10 + 3 + 1 = 14
let me know the OA,
Thanks a lot dude.