AMC 10 Weekly Practice Round 2

Complete problem set with solutions and individual problem pages

Problem 24 Medium

An express train departs from City A toward City B, while a slow train departs from City B toward City A. Both trains depart at the same time and stop upon reaching their destinations. Let the travel time of the slow train be x hours, and let the distance between the two trains be y~\text{km}. The relationship between y and x is shown in the figure. If the two trains meet at a point that is 27~\text{km} away from the midpoint between A and B, then what is the distance between A and B?

  • A.

    270\text{km}

  • B.

    280\text{km}

  • C.

    290\text{km}

  • D.

    300\text{km}

  • E.

    310\text{km}

Answer:A

Let the speed of the express train be a~\text{km/h} and the speed of the slow train be b~\text{km/h}. Let the distance between City A and City B be s~\text{km}. From the graph, we know that the express train takes 6 hours to travel the entire distance, and the slow train takes 9 hours. This gives the equation: 6a = 9b

Since the two trains meet at a point 27~\text{km} away from the midpoint between A and B, we have:\frac{\frac{s}{2} + 27}{a} = \frac{\frac{s}{2} - 27}{b}

By substituting a = \frac{3}{2}b into the equation: \frac{\frac{s}{2} + 27}{\frac{3}{2}b} = \frac{\frac{s}{2} - 27}{b}

Solving gives: s = 270 \ \text{(km)}

Therefore, the answer is 270~\text{km}.