Question 1160418
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The tank volume is  40*30*20 = 24000  cm^3.


The water volume in the tank originally is 50% of this volume.


After submerging the rock into the water, the occupied volume in the tank is 70% of the tank volume.


Hence, to fill the tank, the needed additional water volume is 30% of the tank volume,  i.e.


    0.3*24000 = 7200 cm^3.     <U>ANSWER</U>
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Solved.


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From my post, &nbsp;learn how to get the answer &nbsp;<U>without making unnecessary calculations</U>.