Two boys, each riding a bicycle, start riding towards each other in a straight line from two places 20 kilometers apart. The moment they started off, a fly on the handlebar of one bicycle started flying straight to the other bicycle. As soon as it reached the handlebar of another bicycle, it turned and flew back. The fly goes back and forth in this way, flying back and forth between the handlebars of the two bicycles until the two bicycles meet. If each bicycle moves forward at a constant speed of 10 kilometers per hour, and the fly flies at a constant speed of 15 kilometers per hour, how many kilometers has the fly traveled in total?
两个男孩各骑一辆自行车，从相距20千米的两个地方，开始沿直线相向骑行．在他们起步的那一瞬间，一辆自行车车把上的一只苍蝇，开始向另一辆自行车径直飞去．它一到达另一辆自行车车把，就立即转向往回飞行．这只苍蝇如此往返，在两辆自行车的车把之间来回飞行，直到两辆自行车相遇为止．如果每辆自行车都以每小时10千米的等速前进，苍蝇以每小时15千米的等速飞行，那么，苍蝇总共飞行了多少千米？
<gadget id="calculator">10 + 10</gadget>
<output>20</output>

<gadget id="calculator">20 / 20</gadget>
<output>1</output>

<gadget id="calculator">1 * 15</gadget>
<output>15</output>

<result>15</result>