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100 points for $49 worth of Veritas practice GMATs FREE VERITAS PRACTICE GMAT EXAMS Earn 10 Points Per Post Earn 10 Points Per Thanks Earn 10 Points Per Upvote ## Two trains are heading toward each other on a 750-kilometer tagged by: swerve ##### This topic has 3 expert replies and 0 member replies ### Top Member ## Two trains are heading toward each other on a 750-kilometer Two trains are heading toward each other on a 750-kilometer straight track. One train is heading due east at 70 kilometers per hour and the other train is heading due west at 80 kilometers per hour. How much time do the conductors have before the trains collide? A. 3 hours B. 4 hours C. 5 hours D. 9 hours E. 11 hours The OA is C. Please, can any expert explain this PS question for me? I tried to solve it but I can't get the correct answer. I need your help. Thanks. ### GMAT/MBA Expert GMAT Instructor Joined 04 Dec 2012 Posted: 2094 messages Followed by: 239 members Upvotes: 1443 Top Reply Whenever 2 objects are traveling in OPPOSITE directions (toward each other or away from each other), we can always ADD their rates. Toward: x-----> <-----x The SUM of their individual rates is the rate that the distance between them is CLOSING. Away: <----x x-----> The SUM of their individual rates is the rate that the distance between them is INCREASING. So, these 2 trains will collectively travel the entire 750 miles to meet (crash into) each other at the sum of their individual rates, as EconomistGMAT showed. Another strategy you can easily employ on distance problems: draw a picture! Let's say X is the train going east, Y is the train going west: X|------------------------------------------------|Y 0-----------------------------------------------750 After 1 hr, X has traveled 70 mi, and Y has traveled 80 mi. For Y, count backwards from 750: |-----X------------------------------------Y-------| 0----70--------------------------------670-----750 After 2 hrs: |---------X------------------------------Y-----------| 0-------140------------------------590----------750 After 3 hrs: |--------------X--------------------Y----------------| 0----------210------------------510-------------750 After 4 hrs: |----------------X------------Y------------------| 0--------------280---------430------------750 After 5 hrs: |----------------------XY------------------------| 0---------------------350--------------------750 The answer is C. This technique takes a bit longer, but for some people, drawing it out helps to visualize, and you're less likely to make a mistake. _________________ Ceilidh Erickson Manhattan Prep GMAT & GRE instructor EdM in Mind, Brain, and Education Harvard Graduate School of Education Manhattan Prep instructors all have 99th+ percentile scores and expert teaching experience. Sign up for a FREE TRIAL, and learn why we have the highest ratings in the GMAT industry! Free Manhattan Prep online events - The first class of every online Manhattan Prep course is free. Classes start every week. ### GMAT/MBA Expert GMAT Instructor Joined 04 Oct 2017 Posted: 551 messages Followed by: 11 members Upvotes: 180 Hello swerve. Let's take a look at your question. If we calculate the combine speed we will get 80+70= 150 km/h. Now, the will collide in $$\frac{750km}{150\ \frac{km}{h}}=5\ h.$$ Hence, the correct answer is the option C=15. I hope this answer can help you. I'm available if you'd like a follow-up. Regards. _________________ GMAT Prep From The Economist We offer 70+ point score improvement money back guarantee. Our average student improves 98 points. Free 7-Day Test Prep with Economist GMAT Tutor - Receive free access to the top-rated GMAT prep course including a 1-on-1 strategy session, 2 full-length tests, and 5 ask-a-tutor messages. Get started now. ### GMAT/MBA Expert GMAT Instructor Joined 09 Apr 2015 Posted: 1465 messages Followed by: 19 members Upvotes: 39 swerve wrote: Two trains are heading toward each other on a 750-kilometer straight track. One train is heading due east at 70 kilometers per hour and the other train is heading due west at 80 kilometers per hour. How much time do the conductors have before the trains collide? A. 3 hours B. 4 hours C. 5 hours D. 9 hours E. 11 hours Since the trains are heading toward each other, we can let the time when the two trains would meet be t. Since distance = rate x time, the distance of the train heading east is 70t and the distance of the train heading west is 80t. Now we can determine t: distance of 1st train + distance of 2nd train = total distance 70t + 80t = 750 150t = 750 t = 5 hours Alternate Answer: Since one train travels 70 km an hour and the other travels 80 km an hour, the distance between the trains decreases 70 + 80 = 150 km every hour. Since they were 750 km apart, there will be 750/150 = 5 hours for the conductors to stop them before they collide. Answer: C _________________ Jeffrey Miller Head of GMAT Instruction jeff@targettestprep.com See why Target Test Prep is rated 5 out of 5 stars on BEAT the GMAT. Read our reviews • Free Practice Test & Review How would you score if you took the GMAT Available with Beat the GMAT members only code • 5 Day FREE Trial Study Smarter, Not Harder Available with Beat the GMAT members only code • Magoosh Study with Magoosh GMAT prep Available with Beat the GMAT members only code • FREE GMAT Exam Know how you'd score today for$0

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