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 Your Progress

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.

Online GRE tutoring, like our in-person GRE tutoring, leverages the Official Guide to the GRE Revised General Test. We begin with a diagnostic session to help create a manageable study plan that includes a customized curriculum.

Join a 4-day FREE online boot camp to kick off your GRE preparation and get you into your dream grad school in the upcoming deadlines.**Limited for the first 99 registrants.

This admissions guide will help you plan your best route to a PhD by helping you choose the best programs your goals, secure strong letters of recommendation, strengthen your candidacy, and apply successfully.

Take a free practice test, attend a sample class with one of our highest-rated teachers, or challenge yourself with a GRE® bootcamp. Reserve your seat in a live event. Dates and times vary.

Economist GRE Tutor’s plans all come with a set of practice exams that mirror the official GRE test. Our academic team worked hard to make sure that the practice tests are the closest experience you can get to the real GRE exam. Free 7 day trial

QOTD # 13 Three pumps, P, R, and T, working simultaneously a [#permalink]
10 Sep 2016, 01:43

Expert's post

00:00

Question Stats:

84% (01:27) correct
16% (02:20) wrong based on 25 sessions

Three pumps, P, R, and T, working simultaneously at their respective constant rates, can fill a tank in 5 hours. Pumps P and R, working simultaneously at their respective constant rates, can fill the tank in 7 hours. How many hours will it take pump T, working alone at its constant rate, to fill the tank?

Re: QOTD # 13 Three pumps, P, R, and T, working simultaneously a [#permalink]
11 Sep 2016, 14:01

Expert's post

Carcass wrote:

Three pumps, P, R, and T, working simultaneously at their respective constant rates, can fill a tank in 5 hours. Pumps P and R, working simultaneously at their respective constant rates, can fill the tank in 7 hours. How many hours will it take pump T, working alone at its constant rate, to fill the tank?

A) 1.7 B) 10.0 C) 15.0 D) 17.5 E) 30.0

For work questions, there are two useful rules:

Rule #1: If a person can complete an entire job in k hours, then in one hour, the person can complete 1/k of the job Example: If it takes Sue 5 hours to complete a job, then in one hour, she can complete 1/5 of the job. In other words, her work rate is 1/5 of the job per hour

Rule #2: If a person completes a/b of the job in one hour, then it will take b/a hours to complete the entire job Example: If Sam can complete 1/8 of the job in one hour, then it will take him 8/1 hours to complete the job. Likewise, if Joe can complete 2/3 of the job in one hour, then it will take him 3/2 hours to complete the job.

Let’s use these rules to solve the question. . . .

Three pumps, P, R, and T, working simultaneously at their respective constant rates, can fill a tank in 5 hours So, by Rule #1, pumps P, R and T working together can complete 1/5 of the job in ONE HOUR

Pumps P and R, working simultaneously at their respective constant rates, can fill the tank in 7 hours So, by Rule #1, pumps P, and R working together can complete 1/7 of the job in ONE HOUR

So, pump T's contribution in ONE HOUR = 1/5 - 1/7 = 7/35 - 5/35 = 2/35

So, in ONE HOUR, pump T can complete 2/35 of the job By Rule #2, then, it will take pump T 35/2 hours to complete the ENTIRE JOB

Answer: D

RELATED VIDEO

_________________

Brent Hanneson – Creator of greenlighttestprep.com Sign up for my free GRE Question of the Dayemails