It is known that the distance from point A in the river to Haikou is 60 kilometers. If a boat goes down the current, it will take 4 hours to reach Haikou. The water speed is known to be 6 kilometers per hour. After the boat has sailed for 4 hours, the speed of the water from the sea to the river is 3 kilometers per hour due to the high tide of the sea. How many hours will it take for the boat to sail back to the original place?
已知从河中A地到海口60千米，如船顺流而下，4小时到达海口，已知水速为每小时6千米．船返回已航行4小时后，因海水涨潮，由海向河的水速为每小时3千米，问此船回到原地还需再航行几小时？
<gadget id="calculator">60 / 4</gadget>
<output>15</output>

<gadget id="calculator">15 - 6 - 6</gadget>
<output>3</output>

<gadget id="calculator">3 * 4</gadget>
<output>12</output>

<gadget id="calculator">60 - 12</gadget>
<output>48</output>

<gadget id="calculator">15 - 6</gadget>
<output>9</output>

<gadget id="calculator">9 + 3</gadget>
<output>12</output>

<gadget id="calculator">48 / 12</gadget>
<output>4</output>

<result>4</result>