Savings

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gmattesttaker2: Legendary Member; Posts: 641; Joined: Tue Feb 14, 2012 3:52 pmFollowed by: 8 members; Thanked: 11 times

by gmattesttaker2 » Sat Feb 22, 2014 11:43 am

Hello,

Can you please tell me where I am going wrong here:

If a person can save $380 in 5 weeks, in how many weeks, at this same rate, can the person save 2.6 times this amount?

(A) 13
(B) 12.5
(c) ll
(D) 10.6
(E) 8

OA: A

I am not sure if I am doing some wrong conversions here but I am getting something like 2.6. Can you please assist?

Thanks,
Sri

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Source: Beat The GMAT — Problem Solving | Sat Feb 22, 2014 11:43 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 » Sat Feb 22, 2014 12:19 pm

gmattesttaker2 wrote: If a person can save $380 in 5 weeks, in how many weeks, at this same rate, can the person save 2.6 times this amount?

(A) 13
(B) 12.5
(c) ll
(D) 10.6
(E) 8

A few ways to handle this. Here's one approach:

If it takes 5 weeks to save some amount then, if we want to save 2.6 times the amount, then it will take 2.6 times longer.

(5 weeks)(2.6) = 13 weeks

Answer: A

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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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 » Sat Feb 22, 2014 12:28 pm

gmattesttaker2 wrote:If a person can save $380 in 5 weeks, in how many weeks, at this same rate, can the person save 2.6 times this amount?

(A) 13
(B) 12.5
(c) ll
(D) 10.6
(E) 8

We can also use ratios. It's a little longer, but still pretty fast.

Given: For every 5 weeks, we can save $380

We want to know how many weeks to save (2.6)($380)
So, let x = number of weeks to save (2.6)($380)

Aside: We COULD evaluate (2.6)($380), but we'll save time later on if we don't evaluate it now.

We can use equivalent ratios to compare weeks to savings.
weeks/savings: 5 weeks/380 = x/(2.6)(380)
Cross multiply to get: (5)(2.6)(380) = (x)(380)
Divide both sides by 380 to get: (5)(2.6) = x
Evaluate: [spoiler]13 = x[/spoiler]

Answer: A

Cheers,
Brent

Brent Hanneson - Creator of GMATPrepNow.com

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Spatel: Newbie | Next Rank: 10 Posts; Posts: 7; Joined: Mon Aug 12, 2013 1:13 pm; Thanked: 1 times

by Spatel » Sat Feb 22, 2014 1:22 pm

I used the ratio per week. First figured out how much we are trying to save $380 *2.6 = $988.

Since we already know in 5wks we can have $380 so in 10 wks we'll have $380*2 = $760. Now to get the remaining .6 we can go back to the original ratio of $380 in 5 weeks and find out how much per week of that $380 we are saving ---380/5 = $76 per week.

Now we can take the difference between our original number $988-$760 = 228. We can easily say that $76*3= 228 which would then mean we need to add an additional 3 weeks to our 10 weeks we came up with above.

10+3=13.

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rudzuna: Newbie | Next Rank: 10 Posts; Posts: 4; Joined: Mon Jun 17, 2013 5:22 am

by rudzuna » Sun Mar 09, 2014 6:56 am

This question illustrates to me the principle that GMAT is not testing your math skills but your logic. Doing it the mathematical way, anyone can solve this. A neat way would be:

Calculate how much is saved per week i.e. 380/5 = 76. Then calculate how much you are trying to save which is 380x2.6 = 988. Then simply divide 988/76 to get 13.

Doing it this way is correct but will take the average guy more than 4 minutes between the steps. Remember GMAT questions usually have a solution in 2 or 3 steps only. The first solution offered here is the quickest most logical and will take you 45 seconds max, a time saving which you can then use for harder questions or for double checking your solution. Worst case scenario just work backwards. Logic will tell you anything less than 10 weeks is wrong and you eliminate some of the answer choices.

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Matt@VeritasPrep: GMAT Instructor; Posts: 2630; Joined: Wed Sep 12, 2012 3:32 pmFollowed by: 119 members; Location: East Bay all the way; Thanked: 625 times; GMAT Score:780

by Matt@VeritasPrep » Sun Mar 09, 2014 12:16 pm

Well said, Rudzuna! The basic principles behind any GMAT question you see on test day:

(1) If it looks easy, it isn't - there's some sort of trap. (My mantra: "If I can't see the trap, I'm in the trap.")
(2) If it looks awful, it isn't - there's some sort of trick. (Using the answer choices, picking your own numbers, using units digit properties, etc.)

Computational questions usually have a much easier approach than you'd think at first. Here it's the totally sensible realization that he can save 2.6 times as much money in 2.6 times as much time, so all we have to do is multiply time by 2.6!

For more practice, here's a (much harder, but very fun) follow up question:
https://www.beatthegmat.com/compound-int ... 12277.html

Save $100 on any VP online course!

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[email protected]: Elite Legendary Member; Posts: 10392; Joined: Sun Jun 23, 2013 6:38 pmFollowed by: 511 members; Location: Palo Alto, CA; Thanked: 2867 times; GMAT Score:800

by [email protected] » Sun Mar 09, 2014 9:05 pm

Hi Sri,

A number of posts in this thread have pointed out that the GMAT writers typically craft questions that be solved in a variety of ways. Taking an approach that stresses "lots of math" isn't likely to be the fastest one though. Quant questions (especially DS questions) can often include a "question behind the question"; if you can spot this sub-question, then you'll likely come across a short-cut that will allow you to answer the question faster.

While the GMAT will include a few "simple" questions on everyone's exam, they're not likely to be quite this simple. Here's why:

The $380 mentioned in the prompt is actually irrelevant to the question; any dollar figure (or even a variable) will suffice and the answer won't change. It takes 5 weeks to save "something", so how long does it take to save 2.6 times THAT.....This question is just asking "what's 5 x 2.6?"

Keep your eye open for these types of patterns and you can pick up some easy points (and quickly!).

GMAT assassins aren't born, they're made,
Rich

Contact Rich at [email protected]

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