[
  "Form the equation: 7a + 8b = 200, where a and b are whole numbers and a < b.",
  "Rearrange to 7a = 200 - 8b. Try values of b: 4 gives a=24 (no), 11 gives a=16 (no), 18 gives a=8 (yes).",
  "Difference = 18 - 8 = 10 cm."
]