[
  "For equivalent three-term ratios, find the multiplier that links one known pair of corresponding terms. Apply it to all others.",
  "The middle term changed from 6 to 42: 42 / 6 = 7, so multiply by 7. First: 1 x 7 = 7. Third: 3 x 7 = 21.",
  "The missing terms are 7 and 21. So 1 : 6 : 3 = 7 : 42 : 21."
]