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**KhanDisciple** Thank you for your help, but I don't understand why plugging $\displaystyle (\frac{11}{2})$ (which is what the radius) into the equation $\displaystyle 4\pi r^2\frac{dr}{dt}$ does not work.

The equations $\displaystyle 4\pi r^2\frac{dr}{dt}$ ,and $\displaystyle \frac{1}{6}\pi 3d^2 \frac{dD}{dt}$ look equivalent to me in that they both should solve the problem. In other words, how is the equation $\displaystyle V=\frac{4}{3}\pi r^3$ different from the equation $\displaystyle V=\frac{4}{3}\pi(\frac{diameter}{2})^3$ before and after they are differentiated. Thanks.