A ship sails 21 kilometers downstream and 4 kilometers upstream on the first day. On the second day, sail 12 kilometers downstream and 7 kilometers upstream in the same river. The two times take the same amount of time. Assuming that the speed of the boat itself and the speed of the water flow remain constant, how many times the speed of the boat going with the current is the speed of the boat against the current?
某船第一天顺流航行21千米，逆流航行4千米．第二天在同一河道中顺流航行12千米，逆流航行7千米．两次所用的时间相等．假设船本身速度及水流速度保持不变，顺水船速是逆水船速的多少倍．
<gadget id="calculator">21 - 12</gadget>
<output>9</output>

<gadget id="calculator">7 - 4</gadget>
<output>3</output>

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

<result>3</result>