Two people, A and B, are walking toward each other from two places 36 kilometers apart at the same time. A travels 4 kilometers per hour, and B travels 5 kilometers per hour. If C rides a bicycle and starts in the same direction as A at the same time, and travels 15 kilometers per hour, when he meets B, he immediately turns back and drives towards A, and then turns back and drives towards B after meeting A. This goes back and forth until A and B meet each other. How many kilometers did C travel in total?
甲、乙两人同时从相距36千米的两地相向而行，甲每小时行4千米，乙每小时行5千米．如果丙骑自行车与甲同时同向出发，每小时行15千米，遇到乙后，立即回头向甲驶去，遇到甲后再回头向乙驶去．这样不断来回，直到甲、乙两人相遇为止．问丙共行多少千米？
<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">36 / 9</gadget>
<output>4</output>

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

<result>60</result>