Answer: 400 miles
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Here's one approach to solving
We can form a proportion like so
(120 miles)/(1.5 hours) = (x miles)/(5 hours)
120/1.5 = x/5
120*5 = 1.5*x .................. cross multiply
600 = 1.5x
1.5x = 600
x = 600/1.5
x = 400
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Another approach:
Use the formula
speed = distance/time
to find the train's speed is 120/1.5 = 80 mph
Then we can say,
distance = rate*time
distance = 80*5
distance = 400 miles