A rectangular room measures 12 feet by 15 feet. Carpet costs $4 per square foot. How much will it cost to carpet the entire room?

Multi-step problem: first find the area, then multiply by the unit cost. Area = 12 × 15 = 180 square feet. Cost = 180 × $4 = $720. Common mistake: computing the perimeter (2 × 12 + 2 × 15 = 54 feet) instead of the area — but carpet covers the floor area, not just the perimeter. The perimeter would be relevant for baseboards or molding. Always identify whether the problem asks for: linear measurement (perimeter, length, distance) — uses regular units; area measurement (floor coverage, paint coverage, land area) — uses square units; volume measurement (water, air, soil) — uses cubic units. Real-world applications: flooring/carpet (area × cost per sq ft); paint (area × cost per sq ft, with coverage adjustments for second coats); concrete (volume × cost per cubic yard); gravel (volume × cost per ton). Step-by-step approach to multi-step problems: identify all required calculations, solve each in turn, combine.