Last visit was: 25 Apr 2024, 23:08 It is currently 25 Apr 2024, 23:08

Close
GMAT Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Close
Request Expert Reply
Confirm Cancel
SORT BY:
Date
Tags:
Show Tags
Hide Tags
RC & DI Moderator
Joined: 02 Aug 2009
Status:Math and DI Expert
Posts: 11178
Own Kudos [?]: 31934 [10]
Given Kudos: 290
Send PM
Most Helpful Reply
RC & DI Moderator
Joined: 02 Aug 2009
Status:Math and DI Expert
Posts: 11178
Own Kudos [?]: 31934 [8]
Given Kudos: 290
Send PM
General Discussion
Manager
Manager
Joined: 09 Jan 2016
Status:Persevere
Posts: 107
Own Kudos [?]: 3604 [1]
Given Kudos: 94
Location: Hong Kong
GMAT 1: 750 Q50 V41
GPA: 3.52
Send PM
Manager
Manager
Joined: 09 Jan 2016
Status:Persevere
Posts: 107
Own Kudos [?]: 3604 [0]
Given Kudos: 94
Location: Hong Kong
GMAT 1: 750 Q50 V41
GPA: 3.52
Send PM
Re: If # is an operation which results in adding the digits of integer [#permalink]
chetan2u wrote:
Hi nalin,

you are bang on with answer...

A short and sweet method would involve some number properties..

any consecutive integer will be 1 more than the sum of the digit of smaller number.....
\(11=1+1=2...........12=1+2=3............17=1+7=8............18=1+8=9............19=1+9=10=1+0=1..............20=2+0=2................\)
this will continue till you have SUM as 9 and then it will be 10, but 10 will again give us 1...


so our digits will be 1,2,...7,8,9,1,2,3...9,1.. and so on...
so 90 positive integers will have pattern of 9 digits..

and what does these 9 digits consist of
a) EVEN - 2,4,6,8 -FOUR
b) ODD - 1,3,5,7,9 - FIVE

so picking odd in all 90 will be SAME as picking odd in 9 digits..
ans 5/9


I figured there should be a simpler and faster way.
A suggestion - down the line, if the timer statistics justifies it, you may change the difficulty to above 700 level. I have seen 700 level questions that require lesser application.

Regards,
Nalin
Manager
Manager
Joined: 17 Oct 2015
Posts: 136
Own Kudos [?]: 282 [1]
Given Kudos: 453
Location: India
Concentration: Finance
Schools: ISB '21 (A)
GMAT 1: 690 Q47 V37
GMAT 2: 700 Q44 V41
WE:Corporate Finance (Investment Banking)
Send PM
Re: If # is an operation which results in adding the digits of integer [#permalink]
1
Kudos
Well.. guess im a little late posting the answer.
Was busy figuring out another way to solve this devilish Q.
okay here it goes..

we can look at the cycle of 1 to 9, we got 5 such odd no.s
Similarly 10 to 18, we got another 5 such odds.
Now, 1 to 90 are 90 numbers.. and 10 such 9 consecutive no. sets can be made ((2 of them as illustrated above)) (..90/9=10)

So, the probability shall be ==>(5*10)/(9*10)==>5/9.

Regards.
User avatar
Manager
Manager
Joined: 18 May 2016
Posts: 51
Own Kudos [?]: 106 [0]
Given Kudos: 105
Concentration: Finance, International Business
GMAT 1: 720 Q49 V39
GPA: 3.7
WE:Analyst (Investment Banking)
Send PM
Re: If # is an operation which results in adding the digits of integer [#permalink]
I tried to solve this Q by using the permutations principle. It seems that for any two-digit number to have an ODD sum of digits, its digits must be either both ODD or both EVEN and also the sum must be above more or equal 10.

I tried to solve it somehow along the following lines:
1) Number of applicable one-digit numbers: 5 (1,3,5,7,9)
2) ODD + ODD two-digit numbers: 4 x 5 = 20 (1,3,5,7 & 1,3,5,7,9)
3) ODD + ODD umbers whose digit sum is below 10: 11,13,15,17,31,33,35,51,53,71 or 10 numbers.

However, I never got to the right answer. Is there something flawed with my thinking when it comes to this question? Many thanks for your help! :)
Manager
Manager
Joined: 03 Apr 2013
Posts: 222
Own Kudos [?]: 239 [1]
Given Kudos: 872
Location: India
Concentration: Marketing, Finance
GMAT 1: 740 Q50 V41
GPA: 3
Send PM
Re: If # is an operation which results in adding the digits of integer [#permalink]
1
Kudos
chetan2u wrote:
If # is an operation which results in adding the digits of integer until a single digit is left, what is the probability that a number picked up in first 90 positive integers will have the result of # as an odd digit ?
( example 99 = 9+9 = 18=1+8=9, so #=9)

(a) 4/10
(b) 4/9
(c) 1/2
(d) 6/10
(e) 5/9


self Made


Okay..I really don't think this can be done under the time pressure..but it's a SUPER GOOD question!
I first tried to manually find numbers and find patterns as nalin has done..but no joy..Then I tried to find something related to the digit sum and divisibility by 9..and Bang! Here goes my 2 cents..

The number of possible remainders for 9 is = 9 (0,1,2,3,4,5,6,7,8)
A number whose digit sum is divisible by 9, is itself divisible by 9(Divisibility Rule for 9).
Moreover, both the number and its digit sum leave the same remainder when divided by 9.

Now lets try some numbers..
Remainder 0
9,18,27,36,...81,90

The digit sum is = 9..So we'll count them in. Total numbers of this sort = 10

Remainder 1
10,19,28,37,...82

The digit sum of all these numbers is = 1..So we'll count them in. Total = 9

Remainder 2
11,20,29,38,...83.

Do you see it? all have digit sum = 2..So we'll not count them in.

With similar reasoning..the numbers that leave remainders of 0,1,3,5,7 have to be counted..along with the single digit numbers 1,3,5 and 7. The total comes out to be 50 such numbers.
Probability = 50/90 = 5/9

Answer (E)
Senior Manager
Senior Manager
Joined: 13 Oct 2016
Posts: 300
Own Kudos [?]: 768 [1]
Given Kudos: 40
GPA: 3.98
Send PM
Re: If # is an operation which results in adding the digits of integer [#permalink]
1
Bookmarks
Good question.

First, every multiple of 9 has its digital root (consecutive summation of digits) equal to 9.

So we have arithmetic progression of multiples of 9 to start with: 9, 18, 27 … 90 - total (90-9)/9+1=10

Digital root of every number has a cyclicity of 9. For example let’s start with 1. Digital root of 1 is 1. Next 1+9=10. Digital root of 10 is 1+0=1 still one. Next 10+9=19 => 1+9=10 => 1+0=1 and so on. So we have arithmetic progression 1, 10, 19, … 82. Total (82-1)/9+1=10.

In order for digital root to be odd the numbers in progression should also be odd. We have five odd numbers (1, 3, 5, 7, 9) with 10 elements in progression for each. Total 5*10=50

The sought-for probability will be 50/90 = 5/9

Answer E.

Originally posted by vitaliyGMAT on 31 Oct 2016, 03:03.
Last edited by vitaliyGMAT on 11 Nov 2016, 10:54, edited 3 times in total.
Manager
Manager
Joined: 02 Jul 2016
Posts: 167
Own Kudos [?]: 59 [0]
Given Kudos: 67
Location: India
GMAT 1: 650 Q49 V28
GPA: 4
Send PM
Re: If # is an operation which results in adding the digits of integer [#permalink]
Hey.. can any body tell me what I have done wrong..
I have used a completely different approach..
Since we are given First 90 positive integers and we have to find those numbers whose digit when added gives an ODD integer.
Now here is my reasoning..

We know that
ODD+ EVEN= ODD
OR
EVEN+ODD= ODD
ALSO
if First digit is 9 then the next digit ought to be 0 (9+0=9 (odd) and we are given first 90 numbers) , hence we get our First number

NOW we need to have an ODD and an EVEN digit in order to get an odd number as the sum

CASE 1: ODD+EVEN
We can fill the first position in 4 ways (there are total 5 odd digits and 9 wont be counted)
and the second place as even here we have 5 ways( 5 even digits)
total numbers=4*5=20 numbers

CASE 2: EVEN+ODD
We can fill the first position in 4 ways(zero cant be filled at the first position)
and the second place as ODD as 5 ways( all Odd integers)
Total numbers=4*5=20 numbers

Required number of numbers are 1+20+20=41 numbers
Total given numbers 90

Probablity will be 41/90

CAN ANYBODY PLEASE HELP?????????????????????????????????????? :( :( :( :( :( :( :( :( :( :cry: :cry: :cry:
User avatar
Non-Human User
Joined: 09 Sep 2013
Posts: 32680
Own Kudos [?]: 822 [0]
Given Kudos: 0
Send PM
Re: If # is an operation which results in adding the digits of integer [#permalink]
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
GMAT Club Bot
Re: If # is an operation which results in adding the digits of integer [#permalink]
Moderators:
Math Expert
92915 posts
Senior Moderator - Masters Forum
3137 posts

Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne