Two ferries depart from both sides of the river at the same time, and their respective speeds are fixed. The first meeting is 700 meters away from one bank. After the meeting, continue to move forward and return immediately to the opposite bank (the ferry turning time is not counted). The second meeting is 400 meters away from the other bank. Find the width of the river.
两条渡船分别从江两岸同时开出，它们各自的速度分别是固定的．第一次相遇在距一岸700米处，相遇后继续前进，到对岸立即返回(渡轮转向时间不计)，第二次相遇在距另一岸400米处，求江面的宽．
<gadget id="calculator">700 * 3</gadget>
<output>2_100</output>

<gadget id="calculator">2_100 - 400</gadget>
<output>1_700</output>

<result>1_700</result>