A sand pit is constructed in the shape of a rectangular prism $10 \mathrm{~m}$ long, $50 \mathrm{~cm}$ deep and $2 \mathrm{~m}$ wide. If the pit is already half-full, how much more sand, in $\mathrm{m}^{3}$, is needed to completely fill the pit?
(A) 6
(B) 5
(C) 20
(D) 7.5
(E) 10
The volume of the entire pit is

$$
(10 \mathrm{~m}) \times(50 \mathrm{~cm}) \times(2 \mathrm{~m})=(10 \mathrm{~m}) \times(0.5 \mathrm{~m}) \times(2 \mathrm{~m})=10 \mathrm{~m}^{3}
$$

Since the pit starts with $5 \mathrm{~m}^{3}$ in it, an additional $5 \mathrm{~m}^{3}$ of sand is required to fill it.

ANswer: (B)