Question 987675
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S=speed of slower train; F=speed of faster train=S+5.6mph
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S+F=720.4mph
S+S+5.6mph=720.4mph
2S=714.8mph
S=357.4 
ANSWER 1: The speed of the slower train was 357.4 mph.
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F=S+5.6mph
F=357.4mph+5.6mph=363mph
ANSWER 2: the speed of the faster train was 363 mph.
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CHECK:
S+F=720.4mph
357.4mph+363mph=720.4mph
720.4mph=720.4mph