The formula for the surface of a sphere is $S=4\pi r^2.$ Two lead balls have radii $3$ inches and $6$ inches, resp

The formula for the surface of a sphere is $S=4\pi r^2.$ Two lead balls have radii $3$ inches and $6$ inches, respectively. A third ball has a surface area equal to the sum of the surface areas of the two smaller balls. What is the radius, in inches, of the third ball?

(A) $3\sqrt{10}$
(B) $3\sqrt5$
(C) $36\pi$
(D) $45\pi$
(E) $180\pi$

Answer: B

Source: GMAT Prep

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