Two people, A and B, are retreating at a fixed speed in the space station. At a certain moment, A throws an apple to B at a speed of 1 m/s. After 9 seconds, B receives it and immediately throws it at A at a speed of 1 m/s. 12 seconds After receiving it. Then A immediately throws it to B at a speed of 1 m/s. How many seconds later will A receive it?
甲乙两人在空间站里以固定的速度后退，某时刻甲把一个苹果以1米/秒的速度抛向乙，9秒后乙接到立即又以1米/秒的速度抛向甲，12秒后甲接到．而后甲又立即以1米/秒的速度抛给乙，再过多少秒甲接到？
<gadget id="calculator">12 * 1</gadget>
<output>12</output>

<gadget id="calculator">9 * 1</gadget>
<output>9</output>

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

<gadget id="calculator">3 / 1</gadget>
<output>3</output>

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

<result>15</result>