Convert litres to cubic centimetres, then find how much water stays in the tank.<br><br>
&bull; 5 litres = 5000 cm&sup3;<br>
&bull; Overflow = 800 cm&sup3;<br>
&bull; Water that stays = 5000 − 800 = 4200 cm&sup3;<br>
&bull; Water fills the tank to its full height of 20 cm<br>
&bull; Base area × 20 = 4200<br>
&bull; Base area = 4200 ÷ 20 = 210 cm&sup2;<br><br>
Final Answer: 210 cm&sup2;