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Kristin Fracchia

ACT Challenge Question 20: Total Resistance – Answer and Explanation

Magoosh ACT Challenge Question


Science whizzes! Were you able to solve our ACT Science challenge question this week? If you need a refresher, check out the question and passage here, and then truck on down below for the answer and explanation.


ACT Challenge Question 20

The total resistance in a circuit for resistors connected in series is the sum of their individual resistances. Suppose the student rewired the circuit with a 3.0 ohm and a 6.0 ohm resistor in series, using a 3.0 V battery to supply the voltage. What current would an ammeter placed in this circuit most likely read?

A. 0.333 amps

B. 0.667 amps

C. 1.333 amps

D. 1.500 amps


Answer and Explanation

The correct answer is A.

Applying the definition in the question, we can see that the resistance for a circuit with a total resistance of 3.0 + 6.0 = 9.0 ohms will be the same as it is for a 9.0 ohm resistor alone. Going back to Table 2, we can look up this value as 0.327 amps. Answer choice A is closest to this number. Alternately, you could calculate the current using the formula provided to come up with the exact value. Note that answer choice C is a trap for students who make a bad assumption by simply adding up the measured currents listed in the table for resistors of 3.0 and 6.0 amps.



About Kristin Fracchia

Kristin makes sure Magoosh's blogs are chock-full of awesome, free resources for students preparing for standardized tests. With a PhD from UC Irvine and degrees in Education and English, she’s been working in education since 2004 and has helped students prepare for standardized tests, as well as college and graduate school admissions, since 2007. She enjoys the agonizing bliss of marathon running, backpacking, hot yoga, and esoteric knowledge.

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