Track the fuel amount day by day, then express the remainder as a percentage of the original 400 litres.<br><br>\\(\begin{aligned}<br>\text{Monday: 25\% used} &: 400 \times 0.75 = 300 \text{ litres}<br>\\<br>\text{Tuesday: add 100 L} &: 300 + 100 = 400 \text{ litres}<br>\\<br>\text{Tuesday: 20\% used} &: 400 \times 0.80 = 320 \text{ litres}<br>\\<br>\text{Wednesday: 25\% used} &: 320 \times 0.75 = 240 \text{ litres}<br>\\<br>\text{Percentage of original 400 L} &= \frac{240}{400} \times 100\% = 60\%<br>\end{aligned}\\)<br><br>Final Answer: 60%