Average of y numbers is x

Expert replies

GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Wed Dec 31, 2008 2:24 pm

The average (arithmetic mean) of y numbers is x. If 30 is added to the numbers, then the average (arithmetic mean) will be x-5. What is the value of y in terms of x?

A) x/7 - 5
B) x/6 - 6
C) x/6 - 5
D) x/5 - 7
E) X/5 - 6

Brent Hanneson - Creator of GMATPrepNow.com

Like this post

Source: Beat The GMAT — Problem Solving | Wed Dec 31, 2008 2:24 pm

daisy: Junior | Next Rank: 30 Posts; Posts: 13; Joined: Wed Dec 31, 2008 2:20 pm; Location: London

by daisy » Wed Dec 31, 2008 2:47 pm

IMO (D)

Like this post

cramya: Legendary Member; Posts: 2467; Joined: Thu Aug 28, 2008 6:14 pmFollowed by: 11 members; Thanked: 331 times

by cramya » Wed Dec 31, 2008 2:53 pm

Sum / y = x

Sum = xy

xy+30 / y+1 = x-5

xy+30 = xy+x-5y-5
35 = x-5y

y = x/5 - 7

Choose D)

Like this post

GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Wed Dec 31, 2008 3:10 pm

D it is.
Nice solution, cramya - short and sweet.

Brent Hanneson - Creator of GMATPrepNow.com

Like this post

vivek.kapoor83: Legendary Member; Posts: 833; Joined: Mon Aug 04, 2008 1:56 am; Thanked: 13 times

by vivek.kapoor83 » Thu Jan 01, 2009 2:36 am

hi brent,
Thanks for the gr8 ques..
i was following the same approch as cramya but i couldnt get it
coz in as u said...30 is added to the sum, does it mean total y+1 element will be there...I mean, here we need to treat 30 as next no or something added to already existing y nos.

How we have to treat in exams.

and 1 more query wht is the value of 0^0 in gmac...coz diff explantions are given on diff links...like

https://www.faqs.org/faqs/sci-math-faq/s ... bers/0to0/

Like this post

logitech: Legendary Member; Posts: 2134; Joined: Mon Oct 20, 2008 11:26 pmFollowed by: 25 members; Thanked: 237 times; GMAT Score:730

by logitech » Thu Jan 01, 2009 11:17 am

This question makes me thinking...

How can we solve this problem in different ways ?

if adding 30 is actually decreasing the average by 5.

1) 30 is less than the average of the numbers

For Example we have only 1 number and it is 40

40/1 = 40 ( note that y=1 and x=40)

40+30 / 2 = 35 ( now y =2 and New X = Old X - 5 = 35 )

And also if we look at the correct answer:

X = 5 ( Y + 7 )

I am trying to find out where that 7 is coming into the equation. Since we are adding 30 and it is LESS than the average X > 30 ; so I can see why 7 is not not less than 5.

I am still trying to find out how we can look at this question and SPOT D as the answer by critical thinking.

This is what happens to you when you are an engineer

LGTCH
---------------------
"DON'T LET ANYONE STEAL YOUR DREAM!"

Like this post

logitech: Legendary Member; Posts: 2134; Joined: Mon Oct 20, 2008 11:26 pmFollowed by: 25 members; Thanked: 237 times; GMAT Score:730

by logitech » Thu Jan 01, 2009 11:31 am

Okay check this out

We can always check for y=1 and y=2 and to see if the X will be X-5 when y = 2

IF Y=1

A) x/7 - 5 --> X =42
B) x/6 - 6 --> X =42
C) x/6 - 5 --> X =36
D) x/5 - 7 --> X =40
E) X/5 - 6 --> X =35

Now we can simply add 30 and divide by 2 to see if we will have 5 as a difference

A) X =42 , 36
B) X =42 , 36
C) X =36 , 33
D) X =40 , 35
E) X =35 , 32,5

This calculation can be done within 90 seconds. I like it!

LGTCH
---------------------
"DON'T LET ANYONE STEAL YOUR DREAM!"

Like this post

piyush_nitt: Master | Next Rank: 500 Posts; Posts: 424; Joined: Sun Dec 07, 2008 5:15 pm; Location: Sydney; Thanked: 12 times

by piyush_nitt » Thu Jan 01, 2009 11:58 am

Agree with Vivek.

I did the same mistake , added a 30 to each number and ended up by messing everything.

How someone gonna decide whether number has to be treated as a separate number?

Brant pls explain?

Thanks!

Like this post

GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Thu Jan 01, 2009 12:22 pm

piyush_nitt wrote:Agree with Vivek.

I did the same mistake , added a 30 to each number and ended up by messing everything.

How someone gonna decide whether number has to be treated as a separate number?

Brant pls explain?

Thanks!

Good point. To make this question less ambiguous, I should have written that "30 is added to the set of numbers." That would have been more clear.

Brent Hanneson - Creator of GMATPrepNow.com

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 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