The speed ratio of the car and the truck is 4:7. The two cars are traveling toward each other from two places at the same time and meet at a distance of 15 kilometers from the midpoint. How many kilometers has the train traveled at this time?
汽车和货车的速度比是4：7，两车同时从两地相向而行，在离中点15千米处相遇，这时火车行了多少千米？
<gadget id="calculator">4 + 7</gadget>
<output>11</output>

<gadget id="calculator">7 / 11</gadget>
<output>7/11 = around 0.636364</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(7/11) - (1/2)</gadget>
<output>3/22 = around 0.136364</output>

<gadget id="calculator">15 / (3/22)</gadget>
<output>110</output>

<gadget id="calculator">110 * (7/11)</gadget>
<output>70</output>

<result>70</result>