GekkoJr Posted November 14, 2012 Share Posted November 14, 2012 Hey guys! I failed my GRE with score 159 and I need 160 .... i am very sure i did the following two questions wrong and i have no idea about it ... hope you can help me Queston 1: (simplified) Pumps P1, P2, P3 need 5 hours for a work (i cant remeber for what work, but i think its irrelevant). Pumps P1 and P2 together need 7 hours for the same work. How long would P3 need to to the same work? Question 2: (i am very sure to give you all the information how it was on the test) There is sequence, a1,a2,a3,a(n) ... a1=2 and a2=5 ... a(n) is defined as a(n) = (a(n) - 1) / (a(n) - 2) ... the question was: what is a(135)? maybe i am wrong with numerator or denominator for -1 or -2. But anyway, the solutions were 0,5 or 2 etc ... when i replace a(n) with 135 i did not get any of the solutions ... and i think this solution would be too simple ... because i did not used a1 or a2. Thanks for your help! Quote Link to comment Share on other sites More sharing options...
GRE_Kickboxer Posted November 14, 2012 Share Posted November 14, 2012 Hello, Gekko. The first question can be solved quickly using the formulas seen here: Work Word Problems (with worked solutions) Let me know if that helps you, or if you need more help. For the second question, I'd need to see the original question to make sure you have written it correctly. Regards, Payman Quote Link to comment Share on other sites More sharing options...
GekkoJr Posted November 15, 2012 Author Share Posted November 15, 2012 Thank you for your reply. Unfortunately, i have no further information for question 2. (it was an original gre test question) i am pretty sure, that are all information ... do you have an solution for question 1 for me? thank you for your link ... but i am not able to solve it. I think in the test it was "4 litres are pumped" ... but in my opinion it is irrelevant if we say 4 litres or 10 gallons. but lets say 4 litres, to have something for a calculation thank you! Quote Link to comment Share on other sites More sharing options...
GRE_Kickboxer Posted November 15, 2012 Share Posted November 15, 2012 Sure I can help you. I believe question 1 probably meant what I have written below. I have changed what you wrote to make sure we are discussing the same problem, and to make sure there is no misunderstanding. Pumps P1, P2, P3 together can finish a job in 5 hours. Pumps P1 and P2 together need 7 hours to finish the job. How long would P3 need to finish the job alone? _________________________________________________________ (1 / P1) + (1 / P2) = 1/7 See the formula I gave you the link for. P denotes time for each pump. (1 / P1) + (1 / P2) + (1 / P3) = 1/5 Notice that we now have two equations and that we can subtract the two equations to get rid of "(1 / P1) + (1 / P2)" which will, in turn, leave us with an easy equation to solve: (1 / P1) + (1 / P2) = 1/7 - (1 / P1) + (1 / P2) + (1 / P3) = 1/5 = - (1 / P3) = 1/7 - 1/5 = -(1 / P3) = - 2/35 P3 = 35/2 hours = 17.5 hours By the way, if you need more practice, you might want to check out my quant comparison videos. Payman Quote Link to comment Share on other sites More sharing options...
GekkoJr Posted November 15, 2012 Author Share Posted November 15, 2012 cool, thank you ... know I got it! Quote Link to comment Share on other sites More sharing options...
GRE_Kickboxer Posted November 16, 2012 Share Posted November 16, 2012 No problem. Good luck on your test. Let me know how your studies go. Quote Link to comment Share on other sites More sharing options...
GREQuantum Posted November 20, 2012 Share Posted November 20, 2012 gekkojr, I went ahead and wrote a similar sequence question along the lines of your post. This will keep me out of trouble from ETS. This is an example of a repeating sequence that has a specific pattern. I have attached the image of the problem I wrote. http://www.www.urch.com/forums/attachments/gre-math/6614-need-help-these-2-math-problems-grequestion17.png Here is the link to the video explanation I posted on youtube: Cheers, dabral Quote Link to comment Share on other sites More sharing options...
GREQuantum Posted November 20, 2012 Share Posted November 20, 2012 gekkojr, looks like my post disappeared. i wrote a similar sequence problem, these types of sequences have a repeating pattern, and the key is to identify the pattern. here is the image of the problem statement. [ATTACH=CONFIG]6615[/ATTACH] solve it and then you can watch the video on youtube for the explanation: cheers, dabral Quote Link to comment Share on other sites More sharing options...
greinsanity Posted November 25, 2012 Share Posted November 25, 2012 Can I ask you solve this problem for me step by step? I see a lot of problems like this in practice GRE tests. Is there a universal approach on how to tackle such questions where large amount of numbers is used? Quote Link to comment Share on other sites More sharing options...
greinsanity Posted November 25, 2012 Share Posted November 25, 2012 (edited) Almost manually, I found that the answer is -12. I solved for the first 8 sequences and found a pattern that repeats itself every 6 numbers. Then I took 99/6=16.5 I summed up the first 6 numbers, multiplied them by 16 and added the first three numbers of the sequence. Got -12. Edited November 30, 2012 by greinsanity Quote Link to comment Share on other sites More sharing options...
GREQuantum Posted November 26, 2012 Share Posted November 26, 2012 greinsanity, your method is the best way to do this problem. i don't know if you looked at the video explanation in the link i provided, but it essentially talks about first establishing a pattern and this requires one to compute the first few terms(here 8 terms) and then to find out the number of repeating blocks to finally answer the question. please recognize that this would be a level 5 question(hardest according to ETS) in the exam and someone who scores a 170 will likely spend about three minutes on this question. these are a general type of sequence problems with a repeating pattern. in the exam the test writers will present this same problem with different flavors but the basic approach is the same. dabral Quote Link to comment Share on other sites More sharing options...
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