A room measures 12 feet by 15 feet. If flooring costs $3.50 per square foot, what is the total cost to floor the room?

The area of the room is $12\cdot15=180$ square feet. At $3.50 per square foot, the cost is $180\cdot3.50=630$ dollars, choice D. You can compute it in parts: $180\cdot3=540$ and $180\cdot0.5=90$, and $540+90=630$. Choice C ($472.50) is $135\cdot3.50$, which uses the wrong area. Choice B ($189) is $54\cdot3.50$. Choice A ($94.50) is $27\cdot3.50$, using the sum of the sides, $12+15$, as if it were the area. Cost is based on area, so multiply the two dimensions first.