A student, his home is 30 kilometers away from the school, he goes to school by bicycle every morning, travels at a speed of 15 kilometers per hour, and arrives at school just on time. One morning, because of the headwind, he could only ride at a speed of 10 kilometers per hour for the first 10 kilometers. What speed should he ride for the remaining 20 kilometers to arrive at school on time?
一个学生，他家离学校30千米，他每天早晨骑自行车上学，以每小时15千米的速度行进，恰好准时到校．一天旱晨，因为逆风，开始的10千米，他只能以每小时10千米的速度骑行，剩下的20千米，他应以怎样的速度骑行，才能准时到校？
<gadget id="calculator">30 / 15</gadget>
<output>2</output>

<gadget id="calculator">10 / 10</gadget>
<output>1</output>

<gadget id="calculator">2 - 1</gadget>
<output>1</output>

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

<result>20</result>