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Two trains start at the same moment from stations that are 300 miles apart and travel toward each other on parallel tracks. One train travels at 50 miles per hour and the other at 70 miles per hour. How many hours after they start do the two trains pass each other?
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Correct answer
B
When two objects move toward each other, the gap between them shrinks at the sum of their speeds. The closing speed is 50 + 70 = 120 miles per hour. The time to close the 300-mile gap is 300 ÷ 120 = 2.5 hours.
A common error is averaging the two speeds to 60 and computing 300 ÷ 60 = 5, or using only the slower train's speed for 300 ÷ 50 = 6. Both of these ignore that the gap closes at the combined speed of both trains.
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