The length of the river at the two piers A and B is 90 kilometers, and the water speed is 3 kilometers. Two ships, A and B, set sail from ports A and B at the same time. If they are heading towards each other, they will meet after 3 hours;
A、B两个码头河流长度90千米，水速3千米．甲乙两船分别从A、B两码头同时起航，如果相向而行，3小时后相遇；如果同向而行，15小时甲船追上乙船，那么乙船往返码头一次需要几小时？
<gadget id="calculator">90 / 3</gadget>
<output>30</output>

<gadget id="calculator">90 / 15</gadget>
<output>6</output>

<gadget id="calculator">30 - 6</gadget>
<output>24</output>

<gadget id="calculator">24 / 2</gadget>
<output>12</output>

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

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

<gadget id="calculator">90 / 9</gadget>
<output>10</output>

<gadget id="calculator">6 + 10</gadget>
<output>16</output>

<result>16</result>