This Instructional Activity is a part of the Lesson 21, Unit 5, Grade 8. The purpose of this activity is for students to think about how to find the radius of a sphere when its volume is known. Students can examine the structure of the equation for volume and reason about a number that makes the equation true. They can also notice that Pi is a factor on each side of the equation and divide each side by Pi. Both strategies simplify the solution process and minimize the need for rounding.