Discussion
On decreasing each side of cube by 21%, its surface area decreases by?
Answer: B
The surface area of a cube is \( 6a^{2} \), so it scales with the square of the edge.
\[ a' = 0.79a \quad\Longrightarrow\quad \frac{S'}{S} = (0.79)^{2} = 0.6241 \]
\[ \text{decrease} = 1 - 0.6241 = 0.3759 = 37.59\% \]
The drop is not 42% (twice 21%): the correction term \( \left(\tfrac{21}{100}\right)^{2} = 4.41\% \) must be added back, giving \( 42 - 4.41 = 37.59\% \).
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