GMAT Question of the Day - Daily to your Mailbox; hard ones only

 It is currently 10 Dec 2019, 05:58 ### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

#### Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.  # At his regular hourly rate, Don had estimated the labor cos

Author Message
TAGS:

### Hide Tags

EMPOWERgmat Instructor V
Status: GMAT Assassin/Co-Founder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 15685
Location: United States (CA)
GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: At his regular hourly rate, Don had estimated the labor cos  [#permalink]

### Show Tags

1
Hi paidlukkha,

You can certainly treat this prompt as a 'system' question (2 variables and 2 unique equations.

336 = (X)(H)

However, your second equation is NOT. Since the number of hours increases by 4 and the difference in hourly pay is 2, the equation should be...

336 = (X - 2)(H + 4)

From here, you can proceed with the Algebra and you'll get to the solution.

GMAT assassins aren't born, they're made,
Rich
_________________
Retired Moderator Joined: 29 Apr 2015
Posts: 816
Location: Switzerland
Concentration: Economics, Finance
Schools: LBS MIF '19
WE: Asset Management (Investment Banking)
Re: At his regular hourly rate, Don had estimated the labor cos  [#permalink]

### Show Tags

So you have two equations from the question stem. Let x be the hourly rate and y the time in hours:

$$x*y=336$$

and

$$(y+4)*(x-2) = 336$$ which expands to: $$xy-2y+4x-8=336$$

plug in $$x*y=336$$ in the above formula:

$$336-2y+4x-8=336$$, simplify to:

$$2x-y-2=0$$

Now pick numbers which fit, start with C, then go up to B: 2x-24-4=0, if y=24, then x=14 and 2x=28. So this fits.
Senior Manager  B
Joined: 10 Mar 2013
Posts: 461
Location: Germany
Concentration: Finance, Entrepreneurship
Schools: WHU MBA"20 (A$) GMAT 1: 580 Q46 V24 GPA: 3.88 WE: Information Technology (Consulting) Re: At his regular hourly rate, Don had estimated the labor cos [#permalink] ### Show Tags BrainLab wrote: t(t+4)=672 we can estimate here 20*30=600 --> 22*30=660 So we see that only A or B can be the answer here... 24*28 suits better (22*30) A bit too large numbers to calculate, not really a GMAT Style, though it's an official GMAT Question....... Update We can complete a square here: $$t^2+4t=672$$-> $$t^2+4t+4=672+4$$ --> $$(t+2)^2=26^2$$ T=24, -28 Answer (B) Intern  Joined: 18 Aug 2012 Posts: 9 GMAT 1: 730 Q50 V39 Re: At his regular hourly rate, Don had estimated the labor cos [#permalink] ### Show Tags Given Info: The total hours estimated by Don differs from the actual number of hours required to finish the job. He had to work 4 more hours to finish the job. For this reason, he received$2 less than was estimated to be given per hour for his job. He received a total amount of $336 to complete his job. Interpreting the Problem: We have to find the time Don had initially estimated to complete his job. This can be worked out by forming two equations from 2 different conditions given to us. One can be worked out on the number of hours initially estimated and his hourly rate to complete the job, and the other could be worked out on the actual hours worked nd actual hourly rate he received for the job. After that, we will equate both the equations to the total payment received and find the hours estimated for the job. Solution: Let us assume the time estimated by Don for the job be n hours and let the cost Don charges per hour for the job be$x per hour.

From the information in the question

n Hours (estimated) * x(Don charge for job per hour) = $336 Equation 1: $$nx=336$$ Also, from the information in the question n+4(Hours actualltaken to complete the job)*x-2(Don actual payment per hour)=$336
Equation 2: $$(n+4)(x-2)=336$$

Solving equations 1 and 2 for n and x
Putting nx from equation 1 in equation 2

$$336+4x-2n-8=336$$
$$4x-2n=8$$

Putting value of x in terms of n from Equation 1

$$4(336/n)-2n=8$$
$$672/n-n=4$$
$$n^2+4n-672=0$$
$$n^2+28n-24n-672=0$$
$$(n+28)(n-24)=0$$
$$n=24$$(Ignoring n=-28 as number of hours cannot be negative)

So time Don had estimated to finish the job is 24 hours.
Intern  B
Joined: 08 Feb 2016
Posts: 4
Location: United States
GMAT 1: 700 Q46 V41 GPA: 3.61
WE: Brand Management (Health Care)
Re: At his regular hourly rate, Don had estimated the labor cos  [#permalink]

### Show Tags

3
Reverse Plug in!

1) Start with the middle number and determine in you should go up or down:
336/16= $21/hr 16 is choice C Next add 4 hours -> 336/20 = 16.8. hour Difference is not 2. Move up in hours 2) 336/24 = 14 24 is choice B Next add 4 hours -> 336/28=12 Difference is 2 so this is the answer Current Student D Joined: 12 Aug 2015 Posts: 2549 Schools: Boston U '20 (M) GRE 1: Q169 V154 Re: At his regular hourly rate, Don had estimated the labor cos [#permalink] ### Show Tags Hey Anyone Looking for a more algebraic approach? here is what i actually did let it took x hours to complete the Job So wage per hour = 336/x now as per question => it took x+4 hours to do the job and he was paid => 336/x - 2 /hour now total wage must remain constant hence 336 = (336/x - 2 ) * (x+4) => x^2 +8x - 672 = 0 x=-8+52/2 (neglecting the negative value as hours are non negative ) x= 28 Hence A is sufficient ... Would Love your Thoughts on this approach .. MathRevolution _________________ Director  P Status: Professional GMAT Tutor Affiliations: AB, cum laude, Harvard University (Class of '02) Joined: 10 Jul 2015 Posts: 715 Location: United States (CA) Age: 40 GMAT 1: 770 Q47 V48 GMAT 2: 730 Q44 V47 GMAT 3: 750 Q50 V42 GRE 1: Q168 V169 WE: Education (Education) Re: At his regular hourly rate, Don had estimated the labor cos [#permalink] ### Show Tags This is one of the toughest questions in the OG: 85% incorrect on GMAT Club! I have found that this one is much easier if you just draw a factor tree and use a little bit of trial and error with the various factors (and combinations thereof). Look for one set of numbers that is 4 apart, and the other 2 apart, and you have your answer. You know that the rate and the time are going to be relatively close to one another, because a difference of$2 in Don's hourly rate results in a difference of 4 hours in the time spent on the job. These numbers are not exactly the same, but they are close, suggesting that r and t are relatively close to one another in value.

Attached is a visual that should help.
Attachments Screen Shot 2016-05-26 at 5.33.02 PM.png [ 108.82 KiB | Viewed 2548 times ]

_________________
Harvard grad and 99% GMAT scorer, offering expert, private GMAT tutoring and coaching worldwide since 2002.

One of the only known humans to have taken the GMAT 5 times and scored in the 700s every time (700, 710, 730, 750, 770), including verified section scores of Q50 / V47, as well as personal bests of 8/8 IR (2 times), 6/6 AWA (4 times), 50/51Q and 48/51V.

You can download my official test-taker score report (all scores within the last 5 years) directly from the Pearson Vue website: https://tinyurl.com/y7knw7bt Date of Birth: 09 December 1979.

GMAT Action Plan and Free E-Book - McElroy Tutoring

Contact: mcelroy@post.harvard.edu (I do not respond to PMs on GMAT Club) or find me on reddit: http://www.reddit.com/r/GMATpreparation
Manager  Joined: 18 Jan 2010
Posts: 238
Re: At his regular hourly rate, Don had estimated the labor cos  [#permalink]

### Show Tags

Suppose Don estimated that the job will be completed in t hours.

Then his hourly rate becomes $$\frac{336}{t}$$ $per hour. Now when works for 4 hours longer, his hourly rate is$2 less.
so

$$\frac{336}{t+4}$$ will be the hourly rate

$$\frac{336}{t}$$ - 2 = $$\frac{336}{t+4}$$

t(t+4) = 2 *21*16 = 24 * 28

We get t = 24

VP  P
Joined: 07 Dec 2014
Posts: 1217
Re: At his regular hourly rate, Don had estimated the labor cos  [#permalink]

### Show Tags

algebraic approach with one variable
336/t*(t+4)-2(t+4)=336
t^2+4t-672=0
t=24 hours
Target Test Prep Representative V
Status: Founder & CEO
Affiliations: Target Test Prep
Joined: 14 Oct 2015
Posts: 8656
Location: United States (CA)
Re: At his regular hourly rate, Don had estimated the labor cos  [#permalink]

### Show Tags

1
macjas wrote:
At his regular hourly rate, Don had estimated the labour cost of a repair job as $336 and he was paid that amount. However, the job took 4 hours longer than he had estimated and, consequently, he earned$2 per hour less than his regular hourly rate. What was the time Don had estimated for the job, in hours?

(A) 28
(B) 24
(C) 16
(D) 14
(E) 12

To solve this problem we can translate the problem with the given information into an equation. Since we don’t know Don's hourly rate nor the time he had estimated for the job, we use two variables:

w = Don’s hourly rate

t = number of hours he estimated for the job

We are told that the job took 4 hours longer than he had estimated and, consequently, he earned $2 per hour less than his regular hourly rate, thus $$(r-2)(t+4)=336$$. Complete solution is here: https://gmatclub.com/forum/at-his-regul ... l#p1109300 _________________ Director  D Affiliations: IIT Dhanbad Joined: 13 Mar 2017 Posts: 730 Location: India Concentration: General Management, Entrepreneurship GPA: 3.8 WE: Engineering (Energy and Utilities) Re: At his regular hourly rate, Don had estimated the labor cos [#permalink] ### Show Tags macjas wrote: At his regular hourly rate, Don had estimated the labour cost of a repair job as$336 and he was paid that amount. However, the job took 4 hours longer than he had estimated and, consequently, he earned $2 per hour less than his regular hourly rate. What was the time Don had estimated for the job, in hours? (A) 28 (B) 24 (C) 16 (D) 14 (E) 12 Let the regular hourly rate of Don is x. And the estimated time is y. -> xy =336 Since he took 4 hours longer than the estimated time & was paid the$2 per hour less than his regular hourly rate.
(x-2)(y+4) = 336
-> xy - 2y + 4x - 8 = 336
-> 336 + 4x - 2y - 8 = 336
-> 4x -2y = 8
-> 2x - y = 4
-> 2x - 336/x = 4
-> x - 168/x = 2
-> x^2 - 168 - 2x = 0
-> x^2 - 14x + 12x - 168 = 0
-> x(x-14) + 12(x-14) = 0
-> (x+12)(x-14) = 0
-> x = 14

Estimated time y = 336/14 = 24.

Retired Moderator V
Status: Long way to go!
Joined: 10 Oct 2016
Posts: 1323
Location: Viet Nam
Re: At his regular hourly rate, Don had estimated the labor cos  [#permalink]

### Show Tags

macjas wrote:
At his regular hourly rate, Don had estimated the labour cost of a repair job as $336 and he was paid that amount. However, the job took 4 hours longer than he had estimated and, consequently, he earned$2 per hour less than his regular hourly rate. What was the time Don had estimated for the job, in hours?

(A) 28
(B) 24
(C) 16
(D) 14
(E) 12

$$x$$ is the hours that Don had estimated, so $$\frac{336}{x}$$ is the money per hour he earned.

We have $$(x+4)\times (\frac{336}{x}-2)=336$$
$$\implies (x+4)(336-2x)=336x \\ \implies 336x - 2x^2 + 4\times 336 - 8x = 336x \\ \implies 2x^2 +8x = 4 \times 336 \\ \implies x^2 +4x = 2\times 336 = 672 \\ \implies x^2 + 4x + 4 = 676 \\ \implies (x+2)^2 = 676$$

Note that $$676 = 2\times 338 =2 \times 2 \times 169 = 2^2 \times 13^2 = 26^2$$
Hence $$(x+2)^2=26^2 \implies x+2=26$$ since $$x > 0$$

$$\implies x=24$$. The answer is B.
_________________
Intern  Joined: 20 May 2017
Posts: 4
Re: At his regular hourly rate, Don had estimated the labor cos  [#permalink]

### Show Tags

Hi guys,

I have two questions:

1) I understand every part of the solution, but I need a lot of time for factoring (last step):

(t^2)+4t-672=0 --> factor
(t-24)(t+28)=0

How do you do that? What is your approach?
What I would do is, split 672 up into its factors, which are 2^5*3*7 ... and then I try every single calculation to find the right figures.

2) How can questions like this be done within 2 minutes?
Even after I knew this question by heart, it took me 5 1/2 minutes to get it done, by writing all the important steps down, without taking a break to think.
There are worse questions than this one, but still ...

Thanks,
Tom
GMAT Club Legend  V
Joined: 12 Sep 2015
Posts: 4131
Re: At his regular hourly rate, Don had estimated the labor cos  [#permalink]

### Show Tags

Top Contributor
macjas wrote:
At his regular hourly rate, Don had estimated the labour cost of a repair job as $336 and he was paid that amount. However, the job took 4 hours longer than he had estimated and, consequently, he earned$2 per hour less than his regular hourly rate. What was the time Don had estimated for the job, in hours?

(A) 28
(B) 24
(C) 16
(D) 14
(E) 12

Here's an algebraic solution:

Let h = # of hours that Don ESTIMATED for the job.
So, h + 4 = ACTUAL # of hours it took Don to complete the job.

So, IF Don, had completed the job in h hours, his RATE would have = $336/h However, since Don completed the job in h+4 hours, his RATE was actually =$336/(h + 4)

...consequently, he earned 2$per hour less than his regular hourly rate. In other words, (John's estimated rate) - 2 = (John's actual rate) So,$336/h - 2 = $336/(h + 4) ASIDE: since the above equation is a bit of a pain to solve, you might consider plugging in the answer choices to see which one works. Okay, let's solve this:$336/h - 2 = $336/(h + 4) To eliminate the fractions, multiply both sides by (h)(h+4) to get: 336(h+4) - 2(h)(h+4) = 336h Expand: 336h + 1344 - 2h² - 8h = 336h Simplify: -2h² - 8h + 1344 = 0 Multiply both sides by -1 to get: 2h² + 8h - 1344 = 0 Divide both sides by 2 to get: h² + 4h - 672 = 0 Factor (yeeesh!) to get: (h - 24)(h + 28) = 0 Solve to get: h = 24 or h = -28 Since h cannot be negative (in the real world), h must equal 24. Answer: _________________ Manager  G Joined: 07 Jun 2017 Posts: 160 Location: India Concentration: Technology, General Management GMAT 1: 660 Q46 V38 GPA: 3.6 WE: Information Technology (Computer Software) Re: At his regular hourly rate, Don had estimated the labor cos [#permalink] ### Show Tags Plug the Answer choice always take from middle (C) here 336/16 = 21, so 19*20 not equal to 336 and also it is 380 so go lesser value (B) 336 /24 = 14, S0 12* 26 = 336 fits so answer is B Intern  B Joined: 12 Nov 2015 Posts: 45 Location: United States GRE 1: Q169 V165 Re: At his regular hourly rate, Don had estimated the labor cos [#permalink] ### Show Tags Bunuel wrote: macjas wrote: At his regular hourly rate, Don had estimated the labour cost of a repair job as$336 and he was paid that amount. However, the job took 4 hours longer than he had estimated and, consequently, he earned $2 per hour less than his regular hourly rate. What was the time Don had estimated for the job, in hours? (A) 28 (B) 24 (C) 16 (D) 14 (E) 12 Say the regular hourly rate was $$r$$$ and estimated time was $$t$$ hours, then we would have:

$$rt=336$$ and $$(r-2)(t+4)=336$$;

So, $$(r-2)(t+4)=rt$$ --> $$rt+4r-2t-8=rt$$ --> $$t=2r-4$$.

Now, plug answer choices for $$t$$ and get $$r$$. The pair which will give the product of 336 will be the correct answer.

Answer B fits: if $$t=24$$ then $$r=14$$ --> $$rt=14*24=336$$.

Hope it's clear.

I'm a bit confused here. I see that once I get to t=2r-4 that I can plug in the answers to see which one fits, but If I try answer a, 28, I get 28=2r-4, r=12, 12*28 = 336 which is the same as I get when I plug in answer choice B.

I'm sure I'm missing a step here but I can't figure it out.

Thanks!
Math Expert V
Joined: 02 Sep 2009
Posts: 59632
Re: At his regular hourly rate, Don had estimated the labor cos  [#permalink]

### Show Tags

jboog wrote:
Bunuel wrote:
macjas wrote:
At his regular hourly rate, Don had estimated the labour cost of a repair job as $336 and he was paid that amount. However, the job took 4 hours longer than he had estimated and, consequently, he earned$2 per hour less than his regular hourly rate. What was the time Don had estimated for the job, in hours?

(A) 28
(B) 24
(C) 16
(D) 14
(E) 12

Say the regular hourly rate was $$r$$\$ and estimated time was $$t$$ hours, then we would have:

$$rt=336$$ and $$(r-2)(t+4)=336$$;

So, $$(r-2)(t+4)=rt$$ --> $$rt+4r-2t-8=rt$$ --> $$t=2r-4$$.

Now, plug answer choices for $$t$$ and get $$r$$. The pair which will give the product of 336 will be the correct answer.

Answer B fits: if $$t=24$$ then $$r=14$$ --> $$rt=14*24=336$$.

Hope it's clear.

I'm a bit confused here. I see that once I get to t=2r-4 that I can plug in the answers to see which one fits, but If I try answer a, 28, I get 28=2r-4, r=12, 12*28 = 336 which is the same as I get when I plug in answer choice B.

I'm sure I'm missing a step here but I can't figure it out.

Thanks!

If t = 28, then $$28=2r-4$$ --> $$32=2r$$and $$r = 16$$, not 12.
_________________ Re: At his regular hourly rate, Don had estimated the labor cos   [#permalink] 04 Oct 2017, 03:00

Go to page   Previous    1   2   3   4    Next  [ 62 posts ]

Display posts from previous: Sort by

# At his regular hourly rate, Don had estimated the labor cos  