person takes a sightseeing boat along the downstream direction from port A to port B, and finds that every 40 minutes there is a cargo ship chasing the cruise ship from behind, and every 20 minutes there is a cargo ship passing head-on. It is known that A, The interval between departures of cargo ships between the two ports of B is the same, and the speed of the ships is the same, which is 7 times the speed of water. How many minutes is the interval between departures of cargo ships?
某人乘坐观光船沿顺流方向从A港到B港前行，发现每隔40分钟就有一艘货船从后面追上游船，每隔20分钟就会有一艘货船迎面开过，已知A、B两港之间货船发出间隔时间相同，且船速相同，均是水速的7倍，求货船的发出间隔时间是多少分钟？
<gadget id="calculator">1 / 40</gadget>
<output>1/40 = around 0.025</output>

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

<gadget id="calculator">7 + 1</gadget>
<output>8</output>

<gadget id="calculator">6 / 8</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(3/4) * (1/20)</gadget>
<output>3/80 = around 0.0375</output>

<gadget id="calculator">(1/40) + (3/80)</gadget>
<output>1/16 = around 0.0625</output>

<gadget id="calculator">(1/16) / 2</gadget>
<output>1/32 = around 0.03125</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">1 + (1/7)</gadget>
<output>8/7 = around 1.142857</output>

<gadget id="calculator">(1/32) * (8/7)</gadget>
<output>1/28 = around 0.035714</output>

<gadget id="calculator">1 / (1/28)</gadget>
<output>28</output>

<result>28</result>