Base area of rectangular tank = 40 &times; 25 = 1000 cm&sup2;.<br><br>
&bull; Initial water volume = 6 litres = 6000 cm&sup3;.<br>
&bull; Initial water height = 6000 &divide; 1000 = 6 cm.<br>
&bull; Water poured into cubic tank = 10 &times; 10 &times; 10 = 1000 cm&sup3;.<br>
&bull; Remaining water = 6000 &minus; 1000 = 5000 cm&sup3;.<br>
&bull; Remaining height = 5000 &divide; 1000 = 5 cm.<br>
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Final Answer: 5 cm