Discussion
The sectional density of a bullet of mass 158 g and diameter 0.357 inch is :
Answer: C
The sectional density is about 1239 g/inch2.
Sectional density is the mass of a bullet divided by the square of its diameter, a measure of its ability to penetrate:
\[ \begin{aligned} SD &= \frac{m}{d^{2}} = \frac{158}{(0.357)^{2}} \\ &= \frac{158}{0.12745} \approx 1239.7\ \text{g/inch}^{2} \end{aligned} \]
The value 445 is close to what you get by dividing by the diameter instead of its square (158/0.357 is about 443), and 56 is the product 158 × 0.357. The figures 158 and 0.357 are those of a .357 Magnum bullet, whose weight is normally quoted as 158 grains; the paper writes grams, and the answer follows its units.
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