You can set this up as an equation: \(55x+45x=400\). The 55x is how far the first bus is will have traveled in x hours and the 45x is how far the second bus will have travelled in the same x hours. Since they are traveling in opposite directions, The distances combined should equal 400 mi. That is how I got the equation. You can simplify this equation by adding similar whatcha ma calls its to get: \(100x=400\). Divide 100 from both sides to get \(x=4\). Hope that helps.