Two people are going to explore the desert. They plan to walk 20 kilometers to the depths of the desert every day. It is known that each person can carry at most the food and water that one person plans to walk for 21 days. If two people are going to explore the desert and store some food on the way, if the second person comes back twice as fast as planned, how many kilometers can one of them go deep into the desert? (Of course, the two are required to return to the starting point at the end)
2人要到沙漠中探险，他们计划每天向沙漠深处走20千米，已知每人最多可携带一个人计划速度走21天的食物和水．如果2人要到沙漠中探险、准将部分食物存放在途中，如果第二个人回来速度是计划的2倍，问其中一人最远可以深入沙漠多少千米？(当然要求二人最后返回出发点)
<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<gadget id="calculator">21 + 7</gadget>
<output>28</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 + (1/2)</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">28 / (3/2)</gadget>
<output>56/3 = around 18.666667</output>

<gadget id="calculator">(56/3) * 20</gadget>
<output>1_120/3 = around 373.333333</output>

<result>1_120/3 = around 373.333333</result>