There were 30 seafarers on board an ocean-going ship, and the fresh water on board could be used by all the crew members for 40 days. Ten days after the ship left port, 15 foreign seamen who were killed were rescued on the high seas. If each person uses the same amount of fresh water per day, how many days will the remaining fresh water be used by the people on board?
一艘远洋轮船上共有30名海员，船上的淡水可供全体船员用40天，轮船离港10天后在公海上又救起15名遇难的外国海员．假如每人每天使用的淡水同样多，剩下的淡水可供船上的人再用多少天？
<gadget id="calculator">1 * 30 * 40</gadget>
<output>1_200</output>

<gadget id="calculator">1 * 30 * 10</gadget>
<output>300</output>

<gadget id="calculator">1_200 - 300</gadget>
<output>900</output>

<gadget id="calculator">30 + 15</gadget>
<output>45</output>

<gadget id="calculator">900 / 45</gadget>
<output>20</output>

<result>20</result>