It takes 10 hours for the express train to go from place A to place B, and 15 hours for the slow train to go from place B to place A. The two cars are traveling towards each other from A and B at the same time. After 4 hours, the distance between the two cars is still 200km. The railway between A and B How many kilometers long?
快车从甲地到乙地要行10小时，慢车从乙地到甲地要行15小时，两车同时从甲乙两地相向而行，4小时后两车还相距200km，甲乙两地间的铁路长多少km？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/10) + (1/15)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/6) * 4</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">1 - (2/3)</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">200 / (1/3)</gadget>
<output>600</output>

<result>600</result>