GMAT = Twisted maths?

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SavageDetective: Newbie | Next Rank: 10 Posts; Posts: 6; Joined: Mon Oct 14, 2013 9:19 am

by SavageDetective » Tue Oct 22, 2013 6:49 am

Isn't it true that 460 - 15% = 391?

In question 57 of OG 13 it says that Company P had 15% more employees in December than in January. In December they had 460 employers, how many employees did they have in January?

I used the formula Now=(1-change/100)original

Apparently, answer is B, which contradicts everything I just revised about %'s.

Does anyone care to explain the bizarre logic we should ascribe to in order to answer this question? Nowhere in OG, or GMATPrep Now they cover this in this manner.

Thanks a lot!

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Source: Beat The GMAT — Problem Solving | Tue Oct 22, 2013 6:49 am

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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 » Tue Oct 22, 2013 6:59 am

If the December number is 15% greater than the January number, we cannot also conclude that the January number is 15% less than the December number.

To illustrate this, let's look at a different example.
A = 100
B = 150
Notice that B is 50% greater than A. HOWEVER, A is not 50% less than B. In fact, A is 33 1/3% less than B.

To answer your question, let's let J = # of employees in January.

Company P had 15% more employees in December than in January
In other words, the December number is 15% greater than the January number
So, 460 = 1.15J
Solve to get: J = 460/1.15

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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GMATGuruNY: GMAT Instructor; Posts: 15539; Joined: Tue May 25, 2010 12:04 pmFollowed by: 1906 members; Location: New York, NY; Thanked: 13060 times; GMAT Score:790

by GMATGuruNY » Tue Oct 22, 2013 7:06 am

Company P had 15 percent more employees in December than it had in January. If Company P had 460 employees in December, how many employees did it have in January?

391
400
410
423
445

ALWAYS LOOK AT THE ANSWER CHOICES.

The answer choices here represent the number of employees in January.
There were 15% MORE employees in December.
Thus, 15% of the correct answer choice must yield an integer value.
Only answer choice B is viable.

The correct answer is B.

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SavageDetective: Newbie | Next Rank: 10 Posts; Posts: 6; Joined: Mon Oct 14, 2013 9:19 am

by SavageDetective » Tue Oct 22, 2013 8:20 am

Thanks guys, both methods are extremely helpful.

I must admit I'm getting beaten on %'s for the silliest mistakes.

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SavageDetective: Newbie | Next Rank: 10 Posts; Posts: 6; Joined: Mon Oct 14, 2013 9:19 am

by SavageDetective » Tue Oct 22, 2013 8:22 am

Brent@GMATPrepNow wrote:If the December number is 15% greater than the January number, we cannot also conclude that the January number is 15% less than the December number.

To illustrate this, let's look at a different example.
A = 100
B = 150
Notice that B is 50% greater than A. HOWEVER, A is not 50% less than B. In fact, A is 33 1/3% less than B.

To answer your question, let's let J = # of employees in January.

Company P had 15% more employees in December than in January
In other words, the December number is 15% greater than the January number
So, 460 = 1.15J
Solve to get: J = 460/1.15

Cheers,
Brent

Thanks for the explanation Brent, am definitely going to pay attention to this next time!

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dddanny2006: Master | Next Rank: 500 Posts; Posts: 209; Joined: Thu Jan 12, 2012 12:59 pm

by dddanny2006 » Mon Mar 17, 2014 4:16 pm

Hello Sir,
I initially did make a mistake here resulting in me getting A)391 as the answer.I considered December to be immediately before January rather than Jan-Feb-March-----December

When I thought of it in the latter format,I did understand it much better

But when we think of Jan in terms of being immediately after Dec,the logic does fade away,can you please tell me why is that the case.

In the December of 2013, Company P had 15% more employees than it had this Jan 2014.If company P had 460 employees in the December of 2013,how many employees did it have in January?

460-15/100(460) ==391

Thanks Brent

Brent@GMATPrepNow wrote:If the December number is 15% greater than the January number, we cannot also conclude that the January number is 15% less than the December number.

To illustrate this, let's look at a different example.
A = 100
B = 150
Notice that B is 50% greater than A. HOWEVER, A is not 50% less than B. In fact, A is 33 1/3% less than B.

To answer your question, let's let J = # of employees in January.

Company P had 15% more employees in December than in January
In other words, the December number is 15% greater than the January number
So, 460 = 1.15J
Solve to get: J = 460/1.15

Cheers,
Brent

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