A boat travels 8 kilometers per hour in still water and travels 12 kilometers in 2 hours against the current. What is the speed of the water?
一只船静水中每小时行8千米，逆流行2小时行12千米，水速多少．
<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">8 - 6</gadget>
<output>2</output>

<result>2</result>