There are two places A and B in a river. The estimated speed of a boat is 15 km/h. It takes 4 hours less time to travel from A to B than from B to A. The speed of the water is 3 kilometers per hour. May I ask: How many hours does it take for the boat to go back and forth between A and B?
一条河中有甲乙两地．一条船的预计时速为15千米．它从甲地到达乙地，所用时间比从乙地到达甲地所用时间少4小时，已知水速为每小时3千米，请问：船在甲乙两地往返一共用几小时？
<gadget id="calculator">4 * 15 * 15</gadget>
<output>900</output>

<gadget id="calculator">4 * 15 * 3</gadget>
<output>180</output>

<gadget id="calculator">4 * 3 * 15</gadget>
<output>180</output>

<gadget id="calculator">4 * 3 * 3</gadget>
<output>36</output>

<gadget id="calculator">180 - 180 - 36</gadget>
<output>-36</output>

<gadget id="calculator">900 + (-36)</gadget>
<output>864</output>

<gadget id="calculator">3 - 15</gadget>
<output>-12</output>

<gadget id="calculator">15 + 3 + (-12)</gadget>
<output>6</output>

<gadget id="calculator">15 - 3</gadget>
<output>12</output>

<gadget id="calculator">864 / 6 / 12</gadget>
<output>12</output>

<gadget id="calculator">15 + 3</gadget>
<output>18</output>

<gadget id="calculator">864 / 6 / 18</gadget>
<output>8</output>

<gadget id="calculator">12 + 8</gadget>
<output>20</output>

<result>20</result>