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October 8th, 2008, 12:09 AM  #1 
Newbie Joined: Oct 2008 Posts: 1 Thanks: 0  Need help understanding distance problems
Need help understanding this problem. A freight train starts from Los Angeles and heads for Chicago at 40 mph. Two hours later a passenger train leaves the same station for Chicago traveling at 60 mph. How long will it be before the passenger train overtakes the freight train? If I solve this way: (40)t+2 = 60t t = 4 But, If I solve this way: 40t = (60)t2 t = 6 For some distance problems it doesn't matter and I get the same answer either way, but for this one, I get different answers. An example of getting same answer: Sarah leaves Seattle for New York in her car, averaging 80 mph. One hour later a plane leaves Seattle for New York following the same route and flying 400 mph. How long before the plan overtake the car? (80)t+1 = 400t t= 1/4 or 80t = (400)t1 t = 1/4 My question is why did I get 2 answers, and when solving this type of problem, how do I choose which way of solving is right? thanks 
October 8th, 2008, 04:21 AM  #2 
Global Moderator Joined: Dec 2006 Posts: 20,931 Thanks: 2207 
The first method was correct, but use parentheses correctly: 40(t + 2) = 60t. The second method doesn't work: 80t = 400(t  1) doesn't have solution t = 1/4; it gives t = 5/4, which is incorrect. 

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