Xiao Ming walks to school at a rate of 70 meters per minute. Twelve minutes after leaving home, Dad found that Xiao Ming’s pencil case was left at home. Dad took the pencil case and immediately rode a bicycle to chase Xiao Ming at a speed of 280 meters per minute. So how many minutes after Dad set off, he caught up with Xiao Ming.
小明步行上学，每分钟行70米．离家12分钟后，爸爸发现小明的文具盒忘在家中，爸爸带着文具盒，立即骑自行车以每分钟280米的速度去追小明．那么爸爸出发多少分钟后追上小明．
<gadget id="calculator">70 * 12</gadget>
<output>840</output>

<gadget id="calculator">280 - 70</gadget>
<output>210</output>

<gadget id="calculator">840 / 210</gadget>
<output>4</output>

<result>4</result>