Difficult Average Problem

Expert replies

Previous Topic Next Topic

pkw209: Master | Next Rank: 500 Posts; Posts: 266; Joined: Mon Oct 19, 2009 9:46 pm; Thanked: 8 times; GMAT Score:690

by pkw209 » Wed Apr 14, 2010 4:19 pm

Hi guys,

Could someone please provide an explanation to this problem? Thanks!

a) 85%

b) 240%

c) 350%

d) 480%

e) 580%

Like this post

Source: Beat The GMAT — Problem Solving | Wed Apr 14, 2010 4:19 pm

dxgamez: Senior | Next Rank: 100 Posts; Posts: 65; Joined: Wed Nov 25, 2009 6:33 pm; Thanked: 3 times

by dxgamez » Wed Apr 14, 2010 4:41 pm

Let total sum of numbers for List L be L, total sum of numbers for List M be M.

Qn states :-

(X + Y)/48 = 1.5*(X/43)

solving for Y we get Y = 29x/43

Qn is asking for Ave of (List M/Ave of List L) * 100%

Ave of List M = (29X/43)/5

Ave of List L = X/43

Thus X and 43 are cancelled out for what qn is asking, we are left with:-

(29/5) * 100% = 580%

Ans is E

Like this post

liferocks: Legendary Member; Posts: 576; Joined: Sat Mar 13, 2010 8:31 pmFollowed by: 1 member; Thanked: 97 times

by liferocks » Wed Apr 14, 2010 5:12 pm

so the avaage of numbers in list M is (58-10) or 48 greater then avarage of numbers in list L.
converting this in percentage we get the ans as
[spoiler]D 480%[/spoiler]

Like this post

eaakbari: Master | Next Rank: 500 Posts; Posts: 435; Joined: Mon Mar 15, 2010 6:15 amFollowed by: 1 member; Thanked: 32 times

by eaakbari » Thu Apr 15, 2010 10:28 am

Whats OA

Whether you think you can or can't, you're right.
- Henry Ford

Like this post

pkw209: Master | Next Rank: 500 Posts; Posts: 266; Joined: Mon Oct 19, 2009 9:46 pm; Thanked: 8 times; GMAT Score:690

by pkw209 » Thu Apr 15, 2010 11:43 am

Sorry, I actually don't have the official answer. Can any of the experts please verify?

Thanks.

Like this post

harshavardhanc: Legendary Member; Posts: 526; Joined: Sat Feb 21, 2009 11:47 pm; Location: India; Thanked: 68 times; GMAT Score:680

by harshavardhanc » Thu Apr 15, 2010 12:01 pm

if you say that avg of L is x, you can view the list as 43 X's.

x,x,x,x,x........ 43 times.

Now it is given, the 5 elements of M increase the avg by 50%, i.e each X becomes 1.5X. So, the combined list is

1.5X,1.5X,1.5X,1.5X,1.5X,.......48 times.

That means, the 5 elements in M increase the total by 43* .5 X + 5 * 1.5 X = 29 X . Their avg = (29/5)X.

The original avg is X. Hence, the required % is 29/5 * 100 = 580.

(edited after Stuart's post. Thanks Stuart!)

the increase is : 580 - 100 = 480 %

Last edited by harshavardhanc on Thu Apr 15, 2010 12:06 pm, edited 1 time in total.

Regards,
Harsha

Like this post

Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

by Stuart@KaplanGMAT » Thu Apr 15, 2010 12:02 pm

pkw209 wrote:Hi guys,

Could someone please provide an explanation to this problem? Thanks!

a) 85%

b) 240%

c) 350%

d) 480%

e) 580%

A percent question with unspecified values - let's pick numbers.

To make life simple, let's let list L be {100, 100, 100, ...., 100}, i.e. just 43 100s.

So, the average of L is 100 and the sum of L is 4300.

The average of combined lists L and M is 50% greater, so the combined average is 150.

There are 48 numbers total, so the combined sum is 150*48 = 7200.

48 numbers total means that List M has 5 numbers.

7200 - 4300 = 2900, so those 5 numbers must sum to 2900.

Average = sum/# of terms:

2900/5 = 580

So, M has an average of 580.

Now let's be super careful: we're temped to pick (E) 580%, but the question asks:

"What percent greater than the average..."

To find percent change:

(new amount - old amount)/(old amount) = (580 - 100)/100 = 480

So, the average of M is 480% greater than L: choose (D).

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

Like this post

eaakbari: Master | Next Rank: 500 Posts; Posts: 435; Joined: Mon Mar 15, 2010 6:15 amFollowed by: 1 member; Thanked: 32 times

by eaakbari » Thu Apr 15, 2010 10:06 pm

Stuart Kovinsky wrote:
pkw209 wrote:Hi guys,

Could someone please provide an explanation to this problem? Thanks!

a) 85%

b) 240%

c) 350%

d) 480%

e) 580%
A percent question with unspecified values - let's pick numbers.

To make life simple, let's let list L be {100, 100, 100, ...., 100}, i.e. just 43 100s.

So, the average of L is 100 and the sum of L is 4300.

The average of combined lists L and M is 50% greater, so the combined average is 150.

There are 48 numbers total, so the combined sum is 150*48 = 7200.

48 numbers total means that List M has 5 numbers.

7200 - 4300 = 2900, so those 5 numbers must sum to 2900.

Average = sum/# of terms:

2900/5 = 580

So, M has an average of 580.

Now let's be super careful: we're temped to pick (E) 580%, but the question asks:

"What percent greater than the average..."

To find percent change:

(new amount - old amount)/(old amount) = (580 - 100)/100 = 480

So, the average of M is 480% greater than L: choose (D).

I spent so much time on this question. Thats a brilliant explanation Stuart, you made it seem so easy. But do tell me, when do you use the picking numbers strategy?

Whether you think you can or can't, you're right.
- Henry Ford

Like this post

Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

by Stuart@KaplanGMAT » Thu Apr 15, 2010 10:58 pm

eaakbari wrote:
I spent so much time on this question. Thats a brilliant explanation Stuart, you made it seem so easy. But do tell me, when do you use the picking numbers strategy?

There are a few common situations on the GMAT in which picking numbers is very effective:

1) percent questions with unspecified values.

When you see a % question with unspecified values, you almost always want to pick 100.

2) other questions with unspecified values.

If the choices are fractions, pick the common denominator of all the fractions in the question stem. If not, pick a number that's both permissible (follows the constraints you're given) and manageable (relatively easy to work with).

3) variables in the answer choices and multiple variables in the question.

Pick numbers for the variables that appear in the choices, then work your way through the word problem (or solve the equation) with those numbers.

Then you plug the numbers back into the choices to get a match.

4) odd/even questions.

Since odds and evens follow strict rules, once you pick one set of numbers, you never have to try another.

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

559 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review