Zhangjiazhen Central Primary School is 48 kilometers away from the county seat, part of which is uphill and the rest is downhill. Principal Zhang rode a bicycle from the school to the county seat. It took 4.2 hours to go there and 3.8 hours to return. It is known that Principal Zhang rides his bicycle uphill and travels 10 kilometers per hour, how many kilometers per hour does he ride downhill on his bicycle?
张家镇中心小学距离县城48千米，其中一部分是上坡路，其余是下坡路．张校长骑自行车从学校到县城，去时用了4.2小时，返回时用了3.8小时．已知张校长骑自车上坡每小时行10千米，则他骑自行车下坡每小时行多少千米．
<gadget id="calculator">4.2 + 3.8</gadget>
<output>8</output>

<gadget id="calculator">48 / 10</gadget>
<output>24/5 = around 4.8</output>

<gadget id="calculator">8 - (24/5)</gadget>
<output>16/5 = around 3.2</output>

<gadget id="calculator">48 / (16/5)</gadget>
<output>15</output>

<result>15</result>