It took a ship 4 hours to sail downstream from Pier A to Pier B, and it took 5 hours to go upstream from Pier B to Pier A. If the speed of the current is 3 km/h, what is the speed of the boat in still water?
一艘轮船从甲码头顺流驶向乙码头，用了4小时，从乙码头逆流返回甲码头，用了5小时．已知水流的速度是3千米/时，则船在静水中的速度为多少．
<gadget id="calculator">3 * 5</gadget>
<output>15</output>

<gadget id="calculator">3 * 4</gadget>
<output>12</output>

<gadget id="calculator">15 + 12 + 0</gadget>
<output>27</output>

<gadget id="calculator">5 - 4</gadget>
<output>1</output>

<gadget id="calculator">27 / 1</gadget>
<output>27</output>

<result>27</result>