The distance between the east and the west is 30 kilometers. A and B walk, and C rides a bicycle. A and C go from east to west, and B goes from west to east. The three start at the same time. After C and B meet, C returns immediately, and C's speed remains the same. Go back and forth between A and B, and pass the message until the three meet. If A travels 4 kilometers per hour, B travels 3.5 kilometers per hour, and C travels 15 kilometers per hour. How many kilometers did you travel?
东西两地相距30千米，甲乙两人步行，丙骑自行车，甲和丙由东向西，乙由西向东，三人同时出发，丙与乙相遇后，丙立即返回，丙的速度不变地往返于甲乙两人之间，传递消息直到三人相遇，如果甲每小时行4千米，乙每小时行3.5千米，丙每小时行15千米，从出发到三人相遇丙一共行了多少千米？
<gadget id="calculator">3.5 + 4</gadget>
<output>7.5</output>

<gadget id="calculator">30 / 7.5</gadget>
<output>4</output>

<gadget id="calculator">4 * 15</gadget>
<output>60</output>

<result>60</result>