How many squares with side 1/2 inch long are needed to cover a rectangle that is 4 feet long and 6 feet wide ?

Answer: Option D

length of rectangle = 4ft = (4 ×12) inch = 48 inch
length of rectangle = 6ft = (6 ×12) inch = 72 inch
∴ Number of squares :
$$\begin{array}{rl}& =\frac{48\times 72}{\frac{1}{2}\times \frac{1}{2}}\\ & =13824\end{array}$$