Li Gang works at the school where his son studies. One day, both father and son walked from home to school at the same time. Li Gang walked 20 meters more than his son every minute. After 30 minutes, Li Gang arrived at school and found that he had forgotten his key, so he immediately returned by the same way. I met my son 350 meters away from the school. How many minutes does it take for the son to walk from home to school?
李刚在儿子读书的学校工作．一天，父子二人同时从家步行去学校，李刚每分钟比儿子多走20米，30分钟后李刚到学校，发现忘了带鈅匙，就立即按原路返回．在离校350米的地方遇上儿子．则儿子从家到学校要走多少分钟．
<gadget id="calculator">30 * 20</gadget>
<output>600</output>

<gadget id="calculator">600 - 350</gadget>
<output>250</output>

<gadget id="calculator">350 - 250</gadget>
<output>100</output>

<gadget id="calculator">100 / 20</gadget>
<output>5</output>

<gadget id="calculator">250 / 5</gadget>
<output>50</output>

<gadget id="calculator">350 / 50</gadget>
<output>7</output>

<gadget id="calculator">30 + 5 + 7</gadget>
<output>42</output>

<result>42</result>