The number of natural numbers of two or more than two digits in which digits from left to right are in increasing order is
a.127
b.128
c.502
d.501
e.512
OA: C
Permutation and combination
This topic has expert replies

 Senior  Next Rank: 100 Posts
 Posts: 91
 Joined: 17 Jan 2014
 Thanked: 7 times
 GMATGuruNY
 GMAT Instructor
 Posts: 15497
 Joined: 25 May 2010
 Location: New York, NY
 Thanked: 13060 times
 Followed by:1886 members
 GMAT Score:790
This problem seems a bit too convoluted for the GMAT.parveen110 wrote:The number of natural numbers of two or more than two digits in which digits from left to right are in increasing order is
a.127
b.128
c.502
d.501
e.512
OA: C
Also, the GMAT would say positive integers rather than natural numbers.
That said, the problem does provide some worthwhile takeaways, so here's one approach:
For the digits to increase from left to right, they must be DISTINCT.
For each combination of distinct digits, there is only ONE acceptable ordering: from least to greatest.
For example, only one viable integer can be formed if the selected digits are 7, 1 and 3:
137.
Thus, for the each of the following cases, we need to count how many COMBINATIONS of distinct digits can be formed, since each combination of distinct digits will yield exactly one viable integer.
Note the following constraint:
None of the digits in the following cases can be 0.
The reason:
Since the digits must increase from left to right, any combination that includes 0 must place 0 in the leftmost position  not possible, since the leftmost digit of a positive integer must be at least 1.
Thus, all of the following cases must be formed from the following pool of digits:
1, 2, 3, 4, 5, 6, 7, 8, 9.
We should also note the following:
nCr = nC(nr).
To illustrate:
6C5 = 6C(65) = 6C1.
From 6 options, the number of ways to choose 5 = 6C5 = (6*5*4*3*2)/(5*4*3*2*1) = 6.
From 6 options, the number of ways to choose 1 = 6C1 = 6.
Thus:
6C5 = 6C1.
2digit integers:
Number of ways to choose 2 distinct integers from 9 options = 9C2 = (9*8)/(2*1) = 36.
3digit integers:
Number of ways to choose 3 distinct integers from 9 options = 9C3 = (9*8*7)/(3*2*1) = 84.
4digit integers:
Number of ways to choose 4 distinct integers from 9 options = 9C4 = (9*8*7*6)/(4*3*2*1) = 126.
5digit integers:
Number of ways to choose 5 distinct integers from 9 options = 9C5 = 9C4 = 126.
6digit integers:
Number of ways to choose 6 distinct integers from 9 options = 9C6 = 9C3 = 84.
7digit integers:
Number of ways to choose 7 distinct integers from 9 options = 9C7 = 9C2 = 36.
8digit integers:
Number of ways to choose 8 distinct integers from 9 options = 9C8 = 9C1 = 9.
9digit integers:
Here, there is only 1 option:
123456789.
Total options = 36+84+126+126+84+36+9+1 = 502.
The correct answer is C.
Mitch Hunt
Private Tutor for the GMAT and GRE
GMATGuruNY@gmail.com
If you find one of my posts helpful, please take a moment to click on the "UPVOTE" icon.
Available for tutoring in NYC and longdistance.
For more information, please email me at GMATGuruNY@gmail.com.
Student Review #1
Student Review #2
Student Review #3
Private Tutor for the GMAT and GRE
GMATGuruNY@gmail.com
If you find one of my posts helpful, please take a moment to click on the "UPVOTE" icon.
Available for tutoring in NYC and longdistance.
For more information, please email me at GMATGuruNY@gmail.com.
Student Review #1
Student Review #2
Student Review #3
GMAT/MBA Expert
 Brent@GMATPrepNow
 GMAT Instructor
 Posts: 14456
 Joined: 08 Dec 2008
 Location: Vancouver, BC
 Thanked: 5254 times
 Followed by:1262 members
 GMAT Score:770
Consider the arrangement 123456789parveen110 wrote:The number of natural numbers of two or more than two digits in which digits from left to right are in increasing order is
a.127
b.128
c.502
d.501
e.512
Now PRETEND that each digit has a lightbulb inside of it, and each lightbulb is either on or off.
If the lightbulb is ON, we can see that digit, if that lightbulb is OFF, we cannot see the digit.
So, for example, if we turn on the lightbulbs for digits 2, 5, 7 and 8, and turn off the remaining lightbulbs, we see the resulting number 2578
Notice that the digits are nicely arranged in ascending order. In fact, with this ON/OFF setup, every resulting number will have its digits arranged in ascending order.
So, all we need to do now is count the number different ways to turn on and turn off the indivudual lightbulbs.
Let's take the task of turning the lightbulbs on and off, and break it into stages.
Stage 1: Determine the lightbulb situation for digit 1
There are only two possibilities here: the lightbulb is ON or it's OFF
So, we can complete stage 1 in 2 ways
Stage 2: Determine the lightbulb situation for digit 2
There are only two possibilities here: the lightbulb is ON or it's OFF
So, we can complete stage 2 in 2 ways
Stage 3: Determine the lightbulb situation for digit 3
There are only two possibilities here: the lightbulb is ON or it's OFF
So, we can complete stage 3 in 2 ways
.
.
.
Stage 9: Determine the lightbulb situation for digit 9
There are only two possibilities here: the lightbulb is ON or it's OFF
So, we can complete stage 9 in 2 ways
By the Fundamental Counting Principle (FCP), we can complete all 9 stages (and thus create a number with its digits in ascending order) in (2)(2)(2)(2)(2)(2)(2)(2)(2) ways (= 512 ways
IMPORTANT: Among the 512 possibilities, we have accidentally included some scenarios that don't meet the conditions in the question.
For example, among the 512 possibilities, we have accidentally included 1 scenario where ALL of the lightbulbs are off. In this case, there is NO resulting number.
Likewise, the question tells us that we must have 2 or more digits, and among the 512 possibilities, we have accidentally included 9 scenarios where EXACTLY ONE lightbulb is on (resulting in 1, 2, 3, 4,...8, and 9).
So, the number of numbers that meet the given conditions = 512  1  9
= 502
= C
Cheers,
Brent
Aside: For more information about the FCP, watch our free video: https://www.gmatprepnow.com/module/gmatcounting?id=775
Last edited by Brent@GMATPrepNow on Thu Feb 13, 2014 6:30 am, edited 1 time in total.
Brent Hanneson  Creator of GMATPrepNow.com
Use my video course along with Beat The GMAT's free 60Day Study Guide
Watch these video reviews of my course
And check out these free resources
Use my video course along with Beat The GMAT's free 60Day Study Guide
Watch these video reviews of my course
And check out these free resources

 Senior  Next Rank: 100 Posts
 Posts: 91
 Joined: 17 Jan 2014
 Thanked: 7 times
Hi Brent,Brent@GMATPrepNow wrote:Consider the arrangement 123456789parveen110 wrote:The number of natural numbers of two or more than two digits in which digits from left to right are in increasing order is
a.127
b.128
c.502
d.501
e.512
Now PRETEND that each digit has a lightbulb inside of it, and each lightbulb is either on or off.
If the lightbulb is ON, we can see that digit, if that lightbulb is OFF, we cannot see the digit.
So, for example, if we turn on the lightbulbs for digits 2, 5, 7 and 8, and turn off the remaining lightbulbs, we see the resulting number 2578
Notice that the digits are nicely arranged in ascending order. In fact, with this ON/OFF setup, every resulting number will have its digits arranged in ascending order.
So, all we need to do now is count the number different ways to turn on and turn off the indivudual lightbulbs.
Let's take the task of turning the lightbulbs on and off, and break it into stages.
Stage 1: Determine the lightbulb situation for digit 1
There are only two possibilities here: the lightbulb is ON or it's OFF
So, we can complete stage 1 in 2 ways
Stage 2: Determine the lightbulb situation for digit 2
There are only two possibilities here: the lightbulb is ON or it's OFF
So, we can complete stage 2 in 2 ways
Stage 3: Determine the lightbulb situation for digit 3
There are only two possibilities here: the lightbulb is ON or it's OFF
So, we can complete stage 3 in 2 ways
.
.
.
Stage 9: Determine the lightbulb situation for digit 9
There are only two possibilities here: the lightbulb is ON or it's OFF
So, we can complete stage 9 in 2 ways
By the Fundamental Counting Principle (FCP), we can complete all 9 stages (and thus create a number with its digits in ascending order) in (2)(2)(2)(2)(2)(2)(2)(2)(2) ways (= 512 ways
IMPORTANT: Among the 512 possibilities, we have accidentally included some scenarios that don't meet the conditions in the question.
For example, among the 512 possibilities, we have accidentally included 1 scenario where ALL of the lightbulbs are off. In this case, there is NO resulting number.
Likewise, the question tells us that we must have 2 or more digits, and among the 512 possibilities, we have accidentally included 9 scenarios where EXACTLY ONE lightbulb is off (resulting in 1, 2, 3, 4,...8, and 9).
So, the number of numbers that meet the given conditions = 512  1  9
= 502
= C
Cheers,
Brent
Aside: For more information about the FCP, watch our free video: https://www.gmatprepnow.com/module/gmatcounting?id=775
Brilliant approach,as usual..but in the solution you suggested, are you meaning to say that "we have accidentally included 9 scenarios where EXACTLY ONE lightbulb is 'ON' (resulting in 1, 2, 3, 4,...8, and 9)?? Thanks.
GMAT/MBA Expert
 Brent@GMATPrepNow
 GMAT Instructor
 Posts: 14456
 Joined: 08 Dec 2008
 Location: Vancouver, BC
 Thanked: 5254 times
 Followed by:1262 members
 GMAT Score:770
Good catch, parveen110, thanks!parveen110 wrote:
Hi Brent,
Brilliant approach,as usual..but in the solution you suggested, are you meaning to say that "we have accidentally included 9 scenarios where EXACTLY ONE lightbulb is 'ON' (resulting in 1, 2, 3, 4,...8, and 9)?? Thanks.
I've edited my solution accordingly.
Cheers,
Brent
Brent Hanneson  Creator of GMATPrepNow.com
Use my video course along with Beat The GMAT's free 60Day Study Guide
Watch these video reviews of my course
And check out these free resources
Use my video course along with Beat The GMAT's free 60Day Study Guide
Watch these video reviews of my course
And check out these free resources

 Legendary Member
 Posts: 518
 Joined: 12 May 2015
 Thanked: 10 times
just wondering how to formulate that digit..i am seeing an explanation but can't understand the 'ask' here. n >= 2 digits does it mean 2 digit or 3 or 4,5,6,7,..keeps going on. what is the limitation that we apply or how best can this be broken? why should we consider 123456789 and not any repetitions witin it. (looks like old post but just stuck with this understanding)
GMAT/MBA Expert
 Brent@GMATPrepNow
 GMAT Instructor
 Posts: 14456
 Joined: 08 Dec 2008
 Location: Vancouver, BC
 Thanked: 5254 times
 Followed by:1262 members
 GMAT Score:770
Can you rephrase your question please.vinsmba wrote:just wondering how to formulate that digit..i am seeing an explanation but can't understand the 'ask' here. n >= 2 digits does it mean 2 digit or 3 or 4,5,6,7,..keeps going on. what is the limitation that we apply or how best can this be broken? why should we consider 123456789 and not any repetitions witin it. (looks like old post but just stuck with this understanding)
Aside: "of two or more than two digits" means "two or more digits" (if that's what you're asking)
Cheers,
Brent
Brent Hanneson  Creator of GMATPrepNow.com
Use my video course along with Beat The GMAT's free 60Day Study Guide
Watch these video reviews of my course
And check out these free resources
Use my video course along with Beat The GMAT's free 60Day Study Guide
Watch these video reviews of my course
And check out these free resources
GMAT/MBA Expert
 Brent@GMATPrepNow
 GMAT Instructor
 Posts: 14456
 Joined: 08 Dec 2008
 Location: Vancouver, BC
 Thanked: 5254 times
 Followed by:1262 members
 GMAT Score:770
Quite possibly.nikhilgmat31 wrote:This question seems too tough for GMAT.
If it is within the scope of the GMAT, it's a 750+ level question.
Cheers,
Brent
Brent Hanneson  Creator of GMATPrepNow.com
Use my video course along with Beat The GMAT's free 60Day Study Guide
Watch these video reviews of my course
And check out these free resources
Use my video course along with Beat The GMAT's free 60Day Study Guide
Watch these video reviews of my course
And check out these free resources