Challenge for all those who want to BEAT THE GMAT

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rijul007: Legendary Member; Posts: 588; Joined: Sun Oct 16, 2011 9:42 am; Location: New Delhi, India; Thanked: 130 times; Followed by:9 members; GMAT Score:720

Challenge for all those who want to BEAT THE GMAT

by rijul007 » Fri Nov 11, 2011 11:58 pm

Combinatorics

Find the number of all 5 digit numbers(in base 10) each of which contains the block 15 and is divisible by 15. (for eg. 31545, 34155,...)

Remainder

Find the least possible value of (a+b), where a,b are positive integers such that
11 divides a + 13b, and
13 divides a + 11b.

All the best!!

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Source: Beat The GMAT — Problem Solving | Fri Nov 11, 2011 11:58 pm

shankar.ashwin: Legendary Member; Posts: 966; Joined: Sat Jan 02, 2010 8:06 am; Thanked: 230 times; Followed by:21 members

by shankar.ashwin » Sat Nov 12, 2011 12:48 am

rijul007 wrote:Combinatorics

Remainder

Find the least possible value of (a+b), where a,b are positive integers such that
11 divides a + 13b, and
13 divides a + 11b.

a + 13 b = 11K1 -> 13a + 169b = 143K1
a + 11b = 13K2 -> 11a + 121b = 143K2

Subtracting we have ; 2a + 48b = 143(K1-K2) -> a + 24b = 143K

Therefore, a+24b would be a multiple of 143.

Since we want a minimum value of a+b, try to maximize value of 'b' in a+24b = 143. (b=6 gives 144, so 'b' can be max 5)

a + 24(5) = 143 -> a =23.

a+b = 28 IMO.

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rijul007: Legendary Member; Posts: 588; Joined: Sun Oct 16, 2011 9:42 am; Location: New Delhi, India; Thanked: 130 times; Followed by:9 members; GMAT Score:720

by rijul007 » Sat Nov 12, 2011 4:15 am

shankar.ashwin wrote:
rijul007 wrote:Combinatorics

Remainder

Find the least possible value of (a+b), where a,b are positive integers such that
11 divides a + 13b, and
13 divides a + 11b.

a + 13 b = 11K1 -> 13a + 169b = 143K1
a + 11b = 13K2 -> 11a + 121b = 143K2

Subtracting we have ; 2a + 48b = 143(K1-K2) -> a + 24b = 143K

Therefore, a+24b would be a multiple of 143.

Since we want a minimum value of a+b, try to maximize value of 'b' in a+24b = 143. (b=6 gives 144, so 'b' can be max 5)

a + 24(5) = 143 -> a =23.

a+b = 28 IMO.

Good Job Shankar!!

What about the 1st ques..??

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shankar.ashwin: Legendary Member; Posts: 966; Joined: Sat Jan 02, 2010 8:06 am; Thanked: 230 times; Followed by:21 members

by shankar.ashwin » Sat Nov 12, 2011 4:22 am

Honestly found it too complicated and didnt even want to try. Is there a simple/easy way of doing it?

rijul007 wrote: Good Job Shankar!!

What about the 1st ques..??

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pm; Location: Milpitas, CA; Thanked: 1854 times; Followed by:523 members; GMAT Score:770

by Anurag@Gurome » Sat Nov 12, 2011 4:32 am

These problems are way beyond the difficulty level of GMAT. To be precise, they are from previous National (India) level maths Olympiad papers. I'm not going to solve them here as plenty of solutions are already available in the web.

For problem 1 refer to the solution of question number 4.

For problem 2 refer to the solution of question number 2.

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rijul007: Legendary Member; Posts: 588; Joined: Sun Oct 16, 2011 9:42 am; Location: New Delhi, India; Thanked: 130 times; Followed by:9 members; GMAT Score:720

by rijul007 » Sat Nov 12, 2011 4:35 am

shankar.ashwin wrote:Honestly found it too complicated and didnt even want to try. Is there a simple/easy way of doing it?
rijul007 wrote: Good Job Shankar!!

What about the 1st ques..??

My solution to this problem involves breaking it up into several cases and sub-cases... (7 cases in all)

Even I'm looking for a quicky method to solve this...

Can any of the experts help..??

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rijul007: Legendary Member; Posts: 588; Joined: Sun Oct 16, 2011 9:42 am; Location: New Delhi, India; Thanked: 130 times; Followed by:9 members; GMAT Score:720

by rijul007 » Sat Nov 12, 2011 4:43 am

Anurag@Gurome wrote:These problems are way beyond the difficulty level of GMAT. To be precise, they are from previous National (India) level maths Olympiad papers. I'm not going to solve them here as plenty of solutions are already available in the web.

For problem 1 refer to the solution of question number 4.

For problem 2 refer to the solution of question number 2.

Thanks...
I was mailed these ques from a prep website for CAT(India)

Are these ques even beyond the 700+ level probs in GMAT..?

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GMAT/MBA Expert

Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pm; Location: Milpitas, CA; Thanked: 1854 times; Followed by:523 members; GMAT Score:770

by Anurag@Gurome » Sat Nov 12, 2011 4:48 am

rijul007 wrote:...Are these ques even beyond the 700+ level probs in GMAT..?

For first question, definitely yes. GMAT is not going to test your patience by giving a such lengthy problem that too with so much details.

The second question is however tests your logical skills. Variant of the same may appear in GMAT.

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user123321: Master | Next Rank: 500 Posts; Posts: 385; Joined: Fri Sep 23, 2011 9:02 pm; Thanked: 62 times; Followed by:6 members

by user123321 » Sat Nov 12, 2011 5:27 am

I agree with Anurag. With the amount of time available to solve a question in GMAT, I dont think they are going to test this type of questions, and also this will give more advantage to people who are from Science background but not for others. The wise decision I might make is skip it