There is a circular lane with a length of 4200 meters. After A car starts from point A for 35 seconds, B car starts from point A in the opposite direction. The two cars meet at point B for the first time. If car B changes direction when it starts, that is Keep the same direction as car A when starting out, then car B will catch up with car A before it completes a lap, and the place where it catches up with car A happens to be at point B. Car B turns back immediately after catching up with Car A, and Car A continues to move forward, then the two cars will meet each other at a distance of 300 meters from point A. Find the speed of car B.
有一条长度为4200米的环形车道，甲车从A点出发35秒后，乙车从A点反向出发，两车在B点第一次迎面相遇，如果乙车出发的时候变换方向，即出发的时候和甲车保持同向，那么乙车将行驶完一圈之前追上甲车，并且追上甲车的地点恰好还在B点．乙车追上甲车之后立刻折返，甲车继续前进，那么两车会在距离A点300米的地方迎面相遇．求乙车的速度．
<gadget id="calculator">4_200 / 2</gadget>
<output>2_100</output>

<gadget id="calculator">2_100 / 4</gadget>
<output>525</output>

<gadget id="calculator">525 * 3</gadget>
<output>1_575</output>

<gadget id="calculator">2_100 - 1_575</gadget>
<output>525</output>

<gadget id="calculator">525 / 35</gadget>
<output>15</output>

<gadget id="calculator">15 / 3</gadget>
<output>5</output>

<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<result>20</result>