There is a cuboid water tank with a water injection hole on the top and a drainage hole on the bottom. After the two holes are opened at the same time, if the water is injected at 30 cubic meters per hour, the water tank can be filled in 7 hours; if the water is injected at 45 cubic meters per hour, the water tank can be filled with Use less than 2.5 hours, so how many cubic meters of water are discharged from the small holes on the bottom surface per hour? (assuming the same hourly displacement)
有一个长方体水箱，上面有一个注水孔，底面有一个排水孔，两孔同时打开后，如果每小时注水30立方米，7小时可以注满水箱；如果每小时注水45立方米，注满水箱可少用2.5小时，那么每小时由底面小孔排出多少立方米的水？(设每小时排水量相同)
<gadget id="calculator">4.5 * 45</gadget>
<output>202.5</output>

<gadget id="calculator">210 - 202.5</gadget>
<output>7.5</output>

<gadget id="calculator">7 - 4.5</gadget>
<output>2.5</output>

<gadget id="calculator">7.5 / 2.5</gadget>
<output>3</output>

<result>3</result>