Shawn Berry’s Criticism of the Official Guide for GMAT Review (2015 Quant Review, Problem Solving #16)

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2015 OG Quant Review PS #16. Three machines, individually, can do a certain job in 4, 5, and 6 hours, respectively.  What is the greatest part of the job that can be done in one hour by two of the machines working together at their respective constant rates?

SCOG 2015QR PS16

Math Lessons: (1) Because the machines are working together, add their rates.  Then the combined rate (indicated by Sigma, Σ) is multiplied by the applicable number of hour(s); (2) In the Sigma-row, see that hours in the denominator and hours in the numerator cancel.  Showing Units is vital to keeping track in R*T=D questions;
(3) Notice that the Official Guide solution is built to work for one hour and is not scalable for a wide variety of hours.  My R*T=D solution is scalable to any number of hours and is exactly what you should write when you practice Rate questions and when you take the GMAT-CAT; (4) See that fractional units, here job/hour, indicate a Rate question.  Many words indicate fractional units: “per”, “each”, “every”, “for”, “a”, as in $2 per coffee, $2 each coffee, $2 every coffee, $2 for coffee, and $2 a coffee; (4) In due time, you can simply write R * T = D, , , j (for job), and hr., halving your characters and time spent writing; and (5) One-fourth of Problem Solving questions are Rate * Time = Distance.  Learning the variations of R*T=D is vital to scoring 700+ on the GMAT.

Character count: The OG writes 263 characters but practically no Math, failing to teach you what to write to solve GMAT questions.  Shawn Berry writes 3 lines and 83 characters.  The OG solution uses 317% as many characters.

There’s Joy in Repetition: I will always use the same solution, R*T=D, to solve Rate questions.  In contrast, the Official Guide for GMAT Review writes an ungodly number of different solution methods for Rate questions.

Shawn Berry (550 level). If two machines complete a job in 4 and 5 hours, respectively, and each machine must complete each job independently, how many hours will the machines take, working together, to complete 3 jobs?

A. 6 2/3
B. 7
C. 7 1/3
D. 7 2/3
E. 8

 

Shawn Berry (600 level). If two machines complete a job in 4.5 and 5 hours, respectively, how many hours will the machines take, working together, to complete 3 jobs?

A. 7
B. 7 2/19
C. 7 1/9
D. 7 1/5
E. 7 2/9

 

Shawn Berry (700 level). If two machines complete a job in 4 and 5 hours, respectively, and each machine must complete any quarter of a job independently, how many hours will the machines take, working together, to complete 3 jobs?

A. 6 2/3
B. 7
C. 7 1/3
D. 7 1/2
E. 8

Shawn Berry (800 level). Two machines complete a job uninterrupted in 4 and 5 hours, respectively, but each machine takes two 5 minute breaks each hour so that the human attending can take two 5 minute breaks.  In how many hours will the machines take, working together, to complete 3 jobs?

A. 7 1/2
B. 7 2/3
C. 7 3/4
D. 8
E. 8 1/4

 

 

Legal Note: “The Graduate Management Admissions Council (GMAC) firmly believes that the Official Guide for GMAT Review is all that you need to perform your best on the GMAT … and that no additional techniques or strategies are needed to do well.”  I, Shawn Berry, know better.  I have twice earned a perfect 800 on the GMAT-CAT.  I document that the Official Guide writes inconsistent, inefficient, and downright confusing solutions that take longer than the allotted 2 minutes/question.  Herein I make fair use of GMAC copyrighted material – mostly its confusing solutions – for the transformative educational purpose of teaching students the clear, consistent, and efficient Mathematics, Grammar, and Logic needed to answer GMAT questions in less than 2 minutes.

 

 

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