A room measures 15 feet by 12 feet. What is its area in square inches? (1 foot = 12 inches)

First find the area in square feet: $15\cdot12=180$. One foot is $12$ inches, so one square foot is $12\cdot12=144$ square inches. Then $180\cdot144=25{,}920$ square inches, choice D. You can also convert each side first: $180$ inches by $144$ inches gives $25{,}920$. Choice A ($180$) leaves the answer in square feet. Choice B ($1{,}620$) multiplies by $9$, the square-feet-to-square-yards factor. Choice C ($21{,}600$) multiplies by $120$ instead of $144$. Area conversions use the square of the length conversion factor.