[
  "Set up equations: a + b = 15 and 600a + 800b = 720 x 15 = 10,800.",
  "Substitute b = 15 - a into cost equation: 600a + 800(15-a) = 10,800.",
  "Solve: 600a + 12,000 - 800a = 10,800; -200a = -1,200; a = 6, b = 9."
]