The train is 900 meters long, and it took 1.5 minutes to pass by a big tree beside the road, and 3.5 minutes to pass a bridge at the same speed. Find the length of the bridge.
火车长900米，从路旁的一棵大树旁通过用了1.5分钟，以同样的速度通过一座大桥用了3.5分钟．求这座大桥的长度．
<gadget id="calculator">900 / 1.5</gadget>
<output>600</output>

<gadget id="calculator">600 * 3.5</gadget>
<output>2_100</output>

<gadget id="calculator">2_100 - 900</gadget>
<output>1_200</output>

<result>1_200</result>