Agua and Dumb set off from places A and B, which are 90 kilometers apart, at the same time, and cycle back and forth between A and B continuously. It is known that Dumb's cycling speed is 21 kilometers per hour. Agua rides at a speed of 24 kilometers per hour. I would like to ask: How long did it take for the fifth encounter between the two after departure?
阿瓜和阿呆分别从相距90千米的A、B两地同时出发，在A、B之间不断往返骑车．已知阿呆骑车的速度是每小时21千米．阿瓜骑车的速度是每小时24千米．请问：出发后过多长时间两人第五次相遇？
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">90 * 2 * 4</gadget>
<output>720</output>

<gadget id="calculator">90 + 720</gadget>
<output>810</output>

<gadget id="calculator">21 + 24</gadget>
<output>45</output>

<gadget id="calculator">810 / 45</gadget>
<output>18</output>

<result>18</result>