PSAT Math Multiple-Choice Question 499: Answer and Explanation

Question: 499

The power P, in watts, of an electrical circuit can be calculated using the formula , where V is the voltage of the circuit, in volts, and R is the resistance of the circuit, in ohms. Assuming that the voltage does not change, what is the effect on the power when the resistance is multiplied by a factor of 2?

• A. It is divided by 2.
• B. It is divided by 4.
• C. It is multiplied by 2.
• D. It is multiplied by 4.

Correct Answer: A

Explanation:

A

The question asks for the effect on the power when there is a change to the resistance. No values are given for the variables, but the question is about the relationship between variables, so plug in. Make the initial V = 2 and the initial R = 3. The power becomes . Now multiply the value of R by 2 to get a new R of 2(3) = 6. Keep the value of V = 2, as the voltage remains the same. The new power becomes . This value is less than the initial value for power, so eliminate (C) and (D), which indicate an increase in power. The new power is one-half the initial power, so the power was divided by 2. The correct answer is (A).