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If each side of a cube is decreased by 19% , then the decrease in surface area is :
Let the side of the cube be 'a'
$$SA_1=6a^2$$
Now, the side of the cube is decreased by 19%, thus the side of the cube will be 0.81a.
$$SA_2=6\times\left(0.81a\right)^2$$
Decrease in surface area of the cube = $$\dfrac{6a^2-6\left(0.81a\right)^2}{6a^2}\times100=34.39\%$$