Port A and Port B are 147 kilometers apart. It takes 5 hours for a ship to sail from Port A to Port B, which is 30 minutes less than the time it takes to return against the current. Find the average round trip speed of the ship.
甲、乙两港相距147千米，一艘轮船从甲港驶向乙港用了5小时，比逆水返回时少用了30分钟．求这艘轮船往返的平均速度．
<gadget id="calculator">30 / 60</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">5 + 5 + (1/2)</gadget>
<output>21/2 = around 10.5</output>

<gadget id="calculator">2 / (21/2)</gadget>
<output>4/21 = around 0.190476</output>

<gadget id="calculator">147 * (4/21)</gadget>
<output>28</output>

<result>28</result>