There are three cars A, B, and C. Car A travels 70 kilometers per hour, Car B travels 60 kilometers per hour, and Car C travels 50 kilometers per hour. Car A comes from the east city, and cars B and C from the west city go towards each other at the same time. On the way, after car A met car B, it met car C again after half an hour. Find the distance between the east and west cities.
甲乙丙三辆汽车，甲车每小时走70千米，乙车每小时走60千米，丙车每小时是50千米．甲车从东城，乙丙两车从西城同时相向而行．途中甲车与乙车相遇后，经过半小时又与丙车相遇．求东西两城距离．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">70 + 50</gadget>
<output>120</output>

<gadget id="calculator">1 / 120</gadget>
<output>1/120 = around 0.008333</output>

<gadget id="calculator">70 + 60</gadget>
<output>130</output>

<gadget id="calculator">1 / 130</gadget>
<output>1/130 = around 0.007692</output>

<gadget id="calculator">(1/120) - (1/130)</gadget>
<output>1/1_560 = around 0.000641</output>

<gadget id="calculator">(1/2) / (1/1_560)</gadget>
<output>780</output>

<result>780</result>