[
  "Make the common term Y the same number of units in both ratios to form a three-way ratio.",
  "X : Y = 1 : 3 = 3 : 9, and Y : Z = 9 : 7. So X : Y : Z = 3 : 9 : 7. Z = 7 units = 1400 ml, so 1 unit = 1400 / 7 = 200 ml.",
  "X = 3 units = 3 x 200 = 600 ml."
]