Ships A, B, and C set off at the same time in the same direction and sail around a small island with a circumference of 21 kilometers. Ship A travels 5 kilometers per hour, boat B travels 8 kilometers per hour, and boat C travels 15 kilometers per hour. km. Q: How many hours will it take for the three ships to meet again?
甲、乙、丙三船，同时同地同方向出发，围绕周长为21千米的小岛航行，甲船每小时行5千米，乙船每小时行8千米，丙船每小时行15千米．问：三船再相遇需要几小时？
<gadget id="calculator">15 - 8</gadget>
<output>7</output>

<gadget id="calculator">21 / 7</gadget>
<output>3</output>

<gadget id="calculator">8 - 5</gadget>
<output>3</output>

<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<result>21</result>