Stumped by high school math

[Armymutt25A]

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I'm working on random GRE test questions and I thought I had this type licked until my "quick method" got shot down. Here's the problem:

If Steven can mix 20 drinks in 5 minutes, Sue can mix 20 drinks in 10 minutes, and Jack can mix 20 drinks in 15 minutes, how much time will it take all 3 of them working together to mix the 20 drinks?

The answer is 2:44. Previously, my quick method was to average the time spent and then divide by the number of workers. That comes out to 3:20 in this case. Anyone who was in high school this century (or last) wanna show an old dog how this is done? I can't even think of the name of this type of problem in order to Google it.

Side note: Google has to be the best thing to ever hit the internet. I wish we had it when I was in college. Actually, I wish we had had a well developed internet period.

 

[RainbowCloud]

(20/5)x + (20/10)x + (20/15)x = 20
(120+60+40)/30 x = 20
220x=600
x=2.72min=2:43'2, which is almost the 2:44??

the 20/5, 20/10, 20/15 are the speed of each of the people respectively.. x is the time required, i just used speed times time equaling the no. of drinks produced..
hope that makes sense 🙂

 

[VeganSoprano]

Makes perfect sense to me. Thanks!

 

[lmscout]

If you want a really really good study prep guide, I HIGHLY recommend getting the Kaplan math prep...I hadn't done that type of math in years, and I initially scored a 520, and after that I got this book. I spent a couple of months studying for 1-2 hrs a day, retook the test and got a 670!!! 👍 So, I would recommend that you spend the 20-30 dollars, and get the book, it is worth it!!

 

[DreamDVM]

If you don't want to use algebra to figure this out, try thinking of it this way:

Steven can make 4 drinks/minute, Sue can make 2 drinks/minute, and Jack can make 1.3 drinks per minute. (we get that by dividing 20 by the number of drinks they can make in 20 minutes).

That means that all three working together can make 7.3 drinks in one minute. In 2 minutes, they can make 14.6 drinks, and in three they can make 21.9 drinks.

Now, 21.9 is too many drinks and 14.6 is too few, so we know the number of minutes for 20 drinks has to be in between two and three minutes. If you only have one answer between 2 and 3, you're done!

But, 20 drinks can be divided by 7.3 drinks/minute to get 2.73 minutes. The .73 minutes * 60 seconds means 44 seconds... that's 2 min and 44 seconds.

Good luck!
DreamDVM

 

[PrimalMU]

I'm really, REALLY glad I don't have to take the GRE again... that's all I have to say, haha!

The math section is complete BS in my opinion.

 

[Armymutt25A]

Thank you very much. Looking at your solution, I realized that I learned this back in 8th grade (circa 1990). Why can't they have a reality-based test: Contractor 1 can install the network in 4 weeks, Contractor 2 can do it in 6 weeks, and contractor 3 can do it in 8 weeks. How long will it take if they work together? Answer, about 10 years due to contract issues, budgets, and someone higher up changing the priority to having their building wired.

I'll be borrowing my baby sister's books when I head home for a visit, but I'm stuck with what ever I can scrounge from the internet until March.

 

[Armymutt25A]

.

 

[dyachei]

Thank you very much. Looking at your solution, I realized that I learned this back in 8th grade (circa 1990). Why can't they have a reality-based test: Contractor 1 can install the network in 4 weeks, Contractor 2 can do it in 6 weeks, and contractor 3 can do it in 8 weeks. How long will it take if they work together? Answer, about 10 years due to contract issues, budgets, and someone higher up changing the priority to having their building wired.

I'll be borrowing my baby sister's books when I head home for a visit, but I'm stuck with what ever I can scrounge from the internet until March.

Try number2.com. They have GRE prep materials for free

 

[GellaBella]

i also found SAT books to be helpful for re-learning some of the basic math stuff. Their problems are a littler harder because you can use a calculator on the SAT but in terms of teaching the concepts and getting the hang of some of the questions, they were pretty good.

 

[DVMDream]

Try GREBible...great source. Tons of practice with games and practice tests as well. It is just as much as any of the test-prep books and you can download it to your computer. I wish I had found it sooner!! Good Luck!

 

[Armymutt25A]

I went through the number2.com site and another good on is called sparknotes. Has pages and pages of information.

 

[No Imagination]

Thank you very much. Looking at your solution, I realized that I learned this back in 8th grade (circa 1990). Why can't they have a reality-based test: Contractor 1 can install the network in 4 weeks, Contractor 2 can do it in 6 weeks, and contractor 3 can do it in 8 weeks. How long will it take if they work together? Answer, about 10 years due to contract issues, budgets, and someone higher up changing the priority to having their building wired.

Union regulations require contractor 2 to provide coffee for contractor 1 and 3, and contractor 3 has to oversee the other two. Don't forget to add 0.3 weeks per network for furlow.