An empty ornamental fish tank is 50cm long and 35cm wide, and a fake rock mountain with a height of 30cm and a volume of 300cm**3 is placed inside (the height of the fish tank is greater than the height of the fake rock mountain). If the water pipe injects water into the fish tank at a flow rate of 7.5dm**3 per minute, how many minutes will it take to completely submerge the rockery?
一个空的观赏鱼缸长50cm，宽35cm，里面放一块高30cm，体积为300cm**3的假石山(鱼缸高度大于假石山高度)．如果水管以每分钟7.5dm**3的流量向鱼缸内注水，至少需要多少分钟才能将假石山完全淹没？
<gadget id="calculator">50 * 35 * 30</gadget>
<output>52_500</output>

<gadget id="calculator">52_500 - 300</gadget>
<output>52_200</output>

<gadget id="calculator">7.5 * 1_000</gadget>
<output>7_500</output>

<gadget id="calculator">52_200 / 7_500</gadget>
<output>174/25 = around 6.96</output>

<result>174/25 = around 6.96</result>