Math: e8f9e117

ID: e8f9e117 

(SAT Suite Question Bank > Find Questions > Assessment: SAT + Test: Math + Domain: Algebra)

Comment: Use the formula, then consider what the question is asking.

Method 1: An expanded version of Ohm's law might resemble the following:

I (current, in amperes) = V (potential difference, in volts)/R (resistance, in ohms)

The setup provides us with a resistance—500 ohms—a potential difference—6n volts—and a maximum current—0.25 ampere. Solve for n.

(0.25) ≥ (6n)/(500)

125 ≥ 6n

20.8333 ≥ n

If n represents the greatest number of batteries that can be used, then the decimal must be rounded, since there is no such thing as a partial battery. Going any higher would create too great a potential difference and, subsequently, too great a current, so we have to round down, rather than up. The correct answer must be 20. This last step is where many students would miss the question.

Method 2: Plug in the equation to Desmos and graph, perhaps using y in place of n.

The graph clearly shows the correct value for y (or n), but again, you have to consider the context and round down to 20.

Method 3: Because this question relies so heavily on plugging into the given equation, there is virtually no way to solve without using one of the two methods above.

A comment on the Official Explanation/Rationale: The OE incorrectly states that "a current of no more than 0.25 ampere... can be expressed as I < 0.25." The inequality should be ≤ instead, which means the current could get up to 0.25 ampere, but no greater. As written in the OE, the inequality would have to be interpreted to mean that the current could not reach or exceed 0.25 ampere.