It is currently 21 Feb 2018, 15:07

### 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

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# Working alone at its constant rate, machine A can produce 240 pins in

Author Message
TAGS:

### Hide Tags

Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 4897
GPA: 3.82
Working alone at its constant rate, machine A can produce 240 pins in [#permalink]

### Show Tags

10 Jan 2018, 00:02
00:00

Difficulty:

35% (medium)

Question Stats:

68% (00:57) correct 32% (04:02) wrong based on 62 sessions

### HideShow timer Statistics

[GMAT math practice question]

Working alone at its constant rate, machine A can produce 240 pins in x hours. Working alone at its constant rate, machine B can produce 240 pins in y hours. How many hours will it take machines A and B to produce 240 pins, when working together at their respective constant rates?

1) $$\frac{xy}{(x+y)}=4$$
2) $$x=20$$
[Reveal] Spoiler: OA

_________________

MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
Find a 10% off coupon code for GMAT Club members.
“Receive 5 Math Questions & Solutions Daily”
Unlimited Access to over 120 free video lessons - try it yourself

examPAL Representative
Joined: 07 Dec 2017
Posts: 165
Re: Working alone at its constant rate, machine A can produce 240 pins in [#permalink]

### Show Tags

10 Jan 2018, 02:02
MathRevolution wrote:
[GMAT math practice question]

Working alone at its constant rate, machine A can produce 240 pins in x hours. Working alone at its constant rate, machine B can produce 240 pins in y hours. How many hours will it take machines A and B to produce 240 pins, when working together at their respective constant rates?

1) $$\frac{xy}{(x+y)}=4$$
2) $$x=20$$

Since (1) looks like a very technical expression, we'll look for a technical, equation-intensive solution.
This is a Precise approach.

Machine A produces 240/x pins per hour and machine B produces 240/y pins per hour.
Together they produce 240/x + 240/y pins per hour which simplifies to 240(x+y)/xy pins per hour.
Since rate * time = work then time = work/rate
Therefore, to make 240 pins, they need to work for a total of 240 / (240(x+y)/xy) hours.
This simplifies to xy/(x+y) hours.
Therefore (1) is sufficient to answer the question and (2) is not.

_________________

David
Senior tutor at examPAL
Signup for a free GMAT course

Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 4897
GPA: 3.82
Re: Working alone at its constant rate, machine A can produce 240 pins in [#permalink]

### Show Tags

11 Jan 2018, 23:49
=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.

The first step of the VA (Variable Approach) method is to modify the original condition and the question, and then recheck the question.

We have $$\frac{1}{t} = \frac{1}{x} + \frac{1}{y} = \frac{(x+y)}{xy}$$ or $$t = \frac{xy}{(x+y)}$$, where t is the time that machines A and B take to produce 240 pins, when working together. The question asks for $$\frac{xy}{(x+y)}$$.

Thus, only condition 1) is sufficient. Condition 2) is not sufficient as it only gives us information about $$x$$.

_________________

MathRevolution: Finish GMAT Quant Section with 10 minutes to spare
The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy.
Find a 10% off coupon code for GMAT Club members.
“Receive 5 Math Questions & Solutions Daily”
Unlimited Access to over 120 free video lessons - try it yourself

Re: Working alone at its constant rate, machine A can produce 240 pins in   [#permalink] 11 Jan 2018, 23:49
Display posts from previous: Sort by

# Working alone at its constant rate, machine A can produce 240 pins in

 Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.