People A, B, and C go from city A to city B at the same time. A rides a bicycle 12 kilometers faster than B and 15 kilometers faster than C. A travels 3.5 hours and returns immediately after arriving at city B, at a distance of 30 kilometers from city B. Meet B, ask: How long has it been since C met A?
A、B、C三人同时从甲城去乙城，A骑自行车每小时比B快12公里，比C快15公里，A行3.5小时到达乙城后立即返回，在距乙城30公里处和B相遇，问：C行了多长时间和A相遇？
<gadget id="calculator">30 * 2</gadget>
<output>60</output>

<gadget id="calculator">60 / 12</gadget>
<output>5</output>

<gadget id="calculator">5 - 3.5</gadget>
<output>1.5</output>

<gadget id="calculator">30 / 1.5</gadget>
<output>20</output>

<gadget id="calculator">20 * 3.5</gadget>
<output>70</output>

<gadget id="calculator">70 * 2</gadget>
<output>140</output>

<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<gadget id="calculator">20 + 5</gadget>
<output>25</output>

<gadget id="calculator">140 / 25</gadget>
<output>28/5 = around 5.6</output>

<result>28/5 = around 5.6</result>