A few minutes before the ticket check-in for a certain concert, spectators began to line up to enter the arena, and the number of spectators who came every minute was the same. From the start of ticket checking to the disappearance of the waiting line, it takes 50 minutes if 4 entrances are opened at the same time, and 30 minutes if 6 entrances are opened at the same time. Ask how many minutes it will take if 7 entrances are opened at the same time
某演唱会检票前若干分钟就有观众开始排队等候入场，而每分钟来的观众人数一样多．从开始检票到等候队伍消失，若同时开4个入场口需50分钟，若同时开6个入场口则需30分钟．问如果同时开7个入场口需几分钟多少分钟
<gadget id="calculator">4 * 50</gadget>
<output>200</output>

<gadget id="calculator">6 * 30</gadget>
<output>180</output>

<gadget id="calculator">200 - 180</gadget>
<output>20</output>

<gadget id="calculator">50 - 30</gadget>
<output>20</output>

<gadget id="calculator">20 / 20</gadget>
<output>1</output>

<gadget id="calculator">4 * 50 * 1</gadget>
<output>200</output>

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

<gadget id="calculator">200 - 50</gadget>
<output>150</output>

<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<gadget id="calculator">150 / 6</gadget>
<output>25</output>

<result>25</result>