Question 858458
Two trains leave a station traveling east at the same time. The first train is going 48mph, the second one is going 52mph. How long until the first train is 50 miles ahead of the slower train? 
Here's what I think I have:
d1=rt
d1=48mph(t) 
d2=4t
d2=52mph(t) 
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d1 = 48t
d2 = 52t
d2 - d1 = 50 miles
52t - 48t = 50
t = 12.5 hours
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PS  The 1st train is slower.
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Do it like this:
The speeds differ by 4 mi/hr, so the trains separate at 4 mi/hr
50/4 = 12.5 hours.
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Or, the speed of train 2 relative to train 1 is 4 mi/hr