Mianmian rides a bicycle along the path next to the railway at a speed of 13 meters per second. A train is coming towards him at a speed of 87 meters per second. It takes 4 seconds for him to meet and leave. Find the length of the train.
面面以每秒13米的速度沿铁道边的小路骑车，一辆火车以每秒87米的速度迎面开来，从与面面相遇到离开共用4秒，求火车的车长．
<gadget id="calculator">13 + 87</gadget>
<output>100</output>

<gadget id="calculator">100 * 4</gadget>
<output>400</output>

<result>400</result>