A boat travels 21 kilometers per hour down the river. It is known that the distance traveled by the boat in 2 hours downstream is the same as in 3 hours upstream, how many times the speed of the boat is the speed of the water.
一艘船顺流而下每小时行21千米．已知这艘船顺流2小时与逆流3小时所走的路程相等，求船速是水流速度的几倍．
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">21 * (2/3)</gadget>
<output>14</output>

<gadget id="calculator">21 - 14</gadget>
<output>7</output>

<gadget id="calculator">7 / 2</gadget>
<output>7/2 = around 3.5</output>

<gadget id="calculator">21 - (7/2)</gadget>
<output>35/2 = around 17.5</output>

<gadget id="calculator">(35/2) / (7/2)</gadget>
<output>5</output>

<result>5</result>