Let the height of the box be \(h\) cm. Use the change in sand volume.<br><br>
&bull; Base area = 100 × 40 = 4000 cm&sup2;<br>
&bull; Initial sand = 60% full = 0.6 × 4000 × h = 2400h cm&sup3;<br>
&bull; After removing 30 L (30000 cm&sup3;): sand = \(\frac{1}{5}\) full = 0.2 × 4000 × h = 800h cm&sup3;<br>
&bull; 2400h − 30000 = 800h<br>
&bull; 1600h = 30000<br>
&bull; \(h = \frac{30000}{1600} = 18.75\) cm<br><br>
Final Answer: 18.75 cm