In the morning, my mother started to go to work on foot, walking 70 meters per minute. Six minutes later, Dad found that Mom had forgotten to bring her mobile phone, and Dad rode at a speed of 210 meters per minute to chase Mom. After how many minutes can dad catch up with mom?
早上妈妈步行出发上班，每分钟行70米．6分钟后爸爸发现妈妈忘了带手机，爸爸以每分钟210米的速度骑车去追妈妈．经过多少分钟后爸爸能追上妈妈．
<gadget id="calculator">70 * 6</gadget>
<output>420</output>

<gadget id="calculator">210 - 70</gadget>
<output>140</output>

<gadget id="calculator">420 / 140</gadget>
<output>3</output>

<result>3</result>