Work backwards from the 288 litres left in the tank.<br><br>
[[model:questions/solutions/q10423.png]]<br><br>
After washing, 288 L were left. This is 80% of the remainder after draining (since 20% of the remainder was used for washing).<br>
&bull; Remainder after draining = 288 ÷ 80 × 100 = 288 ÷ 0.8 = 360 L<br>
&bull; The tank originally had 500 L, so water drained = 500 − 360 = 140 L<br><br>
140 litres of water were drained.<br>
Check: 500 − 140 = 360; 20% of 360 = 72 L for washing; 360 − 72 = 288 L left. ✓