Track the remaining percentage after each week.<br><br>\\(\begin{aligned}<br>\text{After Week 1 (30\% used)} &: 70\% \text{ remaining}<br>\\<br>\text{Week 2: 25\% of remainder used} &= 0.25 \times 70\% = 17.5\% \text{ of original}<br>\\<br>\text{After Week 2} &: 70\% - 17.5\% = 52.5\% \text{ remaining}<br>\\<br>\text{Week 3: 40\% of remainder used} &= 0.40 \times 52.5\% = 21\% \text{ of original}<br>\\<br>\text{After Week 3} &: 52.5\% - 21\% = 31.5\% = 6.3 \text{ litres}<br>\\<br>\text{Original} &= 6.3 \div 0.315 = 20 \text{ litres}<br>\end{aligned}\\)<br><br>Final Answer: 20 litres