Find the total oil volume after adding 9 litres, then relate to the tank height.<br><br>
&bull; Base area = 30 × 25 = 750 cm&sup2;<br>
&bull; Initial oil = 750 × 8 = 6000 cm&sup3;<br>
&bull; 9 litres added = 9000 cm&sup3;<br>
&bull; Total oil = 6000 + 9000 = 15000 cm&sup3;<br>
&bull; \(\frac{4}{5}\) of tank height corresponds to 15000 cm&sup3;<br>
&bull; Oil height = 15000 ÷ 750 = 20 cm<br>
&bull; \(\frac{4}{5}\) × tank height = 20 cm<br>
&bull; Tank height = 20 × \(\frac{5}{4}\) = 25 cm<br><br>
Final Answer: 25 cm