# SAT Math Multiple Choice Question 579: Answer and Explanation

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**Question: 579**

**9.** Escape velocity is the speed that a traveling object needs to break free of a planet or moon's gravitational field without additional propulsion (for example, without using fuel). The formula used to calculate escape velocity is , where G represents the universal gravitational constant, m is the mass of the body from which the object is escaping, and r is the distance between the object and the body's center of gravity. Which equation represents the value of r in terms of v, G, and m?

- A.
- B.
- C.
- D.

**Correct Answer:** A

**Explanation:**

A

Difficulty: Medium

Category: Passport to Advanced Math / Exponents

Strategic Advice: Don't spend too much time reading the scientific explanation of the equation. Focus on the question at the very endâ€”it's just asking you to solve the equation for r, or in other words to get r on one side of the equation by itself.

Getting to the Answer: Solve for r using inverse operations. First, square both sides of the equation to remove the radical. Then, multiply both sides by r to get the r out of the denominator. Finally, divide both sides by v2.