There is a circular runway of 390 meters. A and B run in the same direction from the same place at the same time. A runs 80 meters per minute. When B reaches 50 meters, he immediately turns back and doubles the speed to continue running. Meet A at the place, how many meters does B run per minute when he just started?
有一环形跑道390米，甲乙二人从同一地点同时同方向进行跑步，甲每分钟跑80米，乙跑到50米时，立即回头跑，并将速度提1倍继续跑，在距出发点150米处与甲相遇，求乙刚出发时每分钟跑多少米？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">2 * (1/2)</gadget>
<output>1</output>

<gadget id="calculator">50 / 1</gadget>
<output>50</output>

<result>50</result>