From A to B, go down the river at 30 kilometers per hour, and return from B at 24 kilometers per hour. Find the velocity of the water.
从甲到乙，顺流而下，每小时行30千米，从乙返回，每小时行24千米．求流水速度．
<gadget id="calculator">30 - 24</gadget>
<output>6</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<result>3</result>