Make the common term Y the same number of units in both ratios to form a three-way ratio.<br><br>
• X : Y = 1 : 3 = 3 : 9<br>
• Y : Z = 9 : 7<br>
• So X : Y : Z = 3 : 9 : 7<br>
• Z = 7 units = 1400 ml<br>
• 1 unit = 1400 ÷ 7 = 200 ml<br>
• X = 3 units = 3 × 200 = 600 ml<br><br>
Final Answer: 600 ml