Find the total floor area, then multiply by the cost per square metre.
• Living room = 24 m2
• Bedroom = 18 m2
• Total area = 24 + 18 = 42 m2
• Cost per m2 = \$12.85
• Total cost = 42 × 12.85
• 42 × 1285 = 53970; with 2 decimal places: \$539.70
Mrs Wong pays \$539.70 to carpet both rooms.
Check: (24 × 12.85) + (18 × 12.85) = 308.40 + 231.30 = 539.70. ✓
Final Answer: 539.70 dollars