The two brothers set off from school and home at the same time, and walked towards each other. The elder brother walks 100 meters per minute. After 5 minutes, the elder brother exceeds the midpoint by 30 meters. At this time, the two brothers are still 20 meters apart. How many meters does the younger brother walk per minute?
兄弟二人同时从学校和家中出发，相向而行．哥哥每分钟行100米，5分钟后哥哥超过中点30米，这时兄弟二人还相距20米，弟弟每分钟行多少米？
<gadget id="calculator">100 * 5</gadget>
<output>500</output>

<gadget id="calculator">30 * 2</gadget>
<output>60</output>

<gadget id="calculator">500 - 60 - 20</gadget>
<output>420</output>

<gadget id="calculator">420 / 5</gadget>
<output>84</output>

<result>84</result>