Two people, A and B, are walking with their backs facing each other on a road parallel to the railway. A travels 36 kilometers per hour by bicycle, and B travels 3.6 kilometers per hour on foot. A train is coming towards A at a constant speed, and the train passes by A. It took 10 seconds, and it took 21 seconds to pass by side B. Ask how long the train is in meters.
甲、乙两人在与铁路平行的马路上背向而行，甲骑车每小时行36千米，乙步行每小时行3.6千米，一列火车匀速向甲驶来，列车在甲旁开过用了10秒钟，而在乙旁开过用了21秒钟，问这列火车的长是多少米．
<gadget id="calculator">10 * 10</gadget>
<output>100</output>

<gadget id="calculator">100 + 21</gadget>
<output>121</output>

<gadget id="calculator">21 - 10</gadget>
<output>11</output>

<gadget id="calculator">121 / 11</gadget>
<output>11</output>

<gadget id="calculator">11 + 10</gadget>
<output>21</output>

<gadget id="calculator">21 * 10</gadget>
<output>210</output>

<result>210</result>