[
  "Let original boys = b. Then original girls = b + 20 (20 more girls than boys).",
  "After enrolment: boys = 1.10b, girls = 1.25(b + 20) = 1.25b + 25.",
  "Set up: (1.25b + 25) - 1.10b = 26. Solve: 0.15b = 1, b = 20/3."
]