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A train leaves north & travels at 85 kil per hr, another train leaves at the same time & travels south at 90 kil per hr. How long will it take before they are 700 kil apart?

"Train A travels north at 85 kilometers per hour"
After t hours, A has traveled 85t kilometers.

"Train B leaves at the same time and travels south at 90 kilometers per hour"
After t hours, B has traveled 90t kilometers.

Use the Pythagorean theorem to calculate the distance between A and B after t hours.
distance = √(85t + 90t)
    = √(t(85 + 90)
    = t√(85 + 90)
    ≅ 123.8t

Solve for t
123.8t = 700
t ≅ 5.655 hours ≅ 5 hours and 39 minutes

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Janet Yang


Word problems are my favorite type of math questions! I would not feel comfortable answering questions that require specialized knowledge (Physics, Statistics, etc.) because I have not studied these in depth.


I tutor students (fifth through twelfth grades) and am a Top Contributor on Yahoo!Answers with over 24,000 math solutions.

Co-author of An Outline of Scientific Writing: For Researchers With English as a Foreign Language.

I have a Bachelor's degree in Applied Mathematics from the University of California at Berkeley, and a Master of Business Administration degree from The Wharton School.

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George White Elementary School. Homework Help program at the Ridgewood Public Library, Ridgewood, NJ. Individual students.

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