Work backwards from 450 ml left. After giving 1/4 to friend, 3/4 remained = 450ml.<br><br>[[model:questions/solutions/q4335.png]]<br><br>• Left 450 = 3/4 after friend → 1/4 = 150, total before friend = 600 ml<br>• After friend 600 = 2/3 after afternoon → 1/3 = 300, total before afternoon = 900 ml<br>• After afternoon 900 = 3/5 of total → 1/5 = 300, total at first = 1500 ml<br><br>Levin had 1500 ml of water at first.<br>Check: Morning 2/5 × 1500 = 600. Remainder = 900. Afternoon 1/3 × 900 = 300. Remainder = 600. Friend 1/4 × 600 = 150. Left = 450. ✓