Atrain leaves little rock , arkansas, and travels north at 70 kilometers per hour . another train leaves at the same times and travel south at 85 kilometers per hour . how long will it take before they are 620 kilometers apart?

distance = rate times time.

Here the distance covered by the first train is (70 km/hr)t (t=# of hours), and that by the second train is (85 km/hr)t.

Add these together and set the sum equal to 620 km:

70t + 85t = 620, or 155t = 620, or 4. It will take 4 hours for the distance between the two trains to increase to 620 km.

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