A small boat sails 48 kilometers downstream and 12 kilometers upstream for the first time, and it takes 9 hours; for the second time, it sails 40 kilometers downstream and 16 kilometers upstream in the same time. speed in water.
一条小船，第一次顺流航行48千米，逆流航行12千米，共用9小时；第二次用同样的时间，顺流航行40千米，逆流航行16千米，求这条小船在静水中的速度．
<gadget id="calculator">48 - 40</gadget>
<output>8</output>

<gadget id="calculator">16 - 12</gadget>
<output>4</output>

<gadget id="calculator">8 / 4</gadget>
<output>2</output>

<gadget id="calculator">12 * 2</gadget>
<output>24</output>

<gadget id="calculator">48 + 24</gadget>
<output>72</output>

<gadget id="calculator">72 / 9</gadget>
<output>8</output>

<gadget id="calculator">48 / 8</gadget>
<output>6</output>

<gadget id="calculator">9 - 6</gadget>
<output>3</output>

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

<gadget id="calculator">8 + 4</gadget>
<output>12</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<result>6</result>