Let original amount = \\(x\\) litres. Work through each step.<br><br>\\(\begin{aligned}<br>\text{After Day 1 (20\% used)} &: 0.80x<br>\\<br>\text{After adding 10 litres} &: 0.80x + 10<br>\\<br>\text{After using 25\%} &: 0.75(0.80x + 10) = 0.60x + 7.5<br>\\<br>\text{Final amount} &= 45 \text{ litres}<br>\\<br>0.60x + 7.5 &= 45<br>\\<br>0.60x &= 37.5<br>\\<br>x &= 37.5 \div 0.60 = 62.5<br>\end{aligned}\\)<br><br>Final Answer: 62.5 litres