Every day at 5 o'clock in the afternoon, Ding Ding leaves school, and his father rides a bicycle to the school gate to pick him up. One day, school ends at 4 o'clock in the afternoon, and Ding Ding walks home at a speed of 3 kilometers per hour. On the way, he meets his father riding a bicycle. I came back, but I got home 20 minutes earlier than usual this day, begging my father for his cycling speed.
每天下午5点钟丁丁放学，爸爸在这时骑车到达校门口接他回家，一天，学校下午4点就放学了，丁丁以每小时3千米的速度步行回家，途中遇见爸爸骑车回来了，结果这一天比往常早20分钟到家，求爸爸的骑车速度．
<gadget id="calculator">3 * 5</gadget>
<output>15</output>

<result>15</result>