Proportions

Read the following SAT test question and then select the correct answer. 

The Knowsys Method is to read the question carefully and identify the bottom line.  Think about your options for solving the problem and choose the most efficient method.  Then attack the problem, solve it, and loop back to make sure that you solved for the bottom line.

A gasoline tank on a certain tractor holds 16 gallons of gasoline. If the tractor requires 7 gallons to plow 3 acres, how many acres can the tractor plow with a tankful of gasoline?

First, find the bottom line.  Your bottom line asks how many acres can be plowed with one tankful of gasoline.  A tankful is 16 gallons and you have information about how many gallons it takes to plow a certain number of acres.  Consider your options: you could use basic arithmetic to estimate the answer, you could solve for one gallon of gasoline and then multiply it by 16, or you could use a proportion.  Use a proportion because it only requires you to set up one problem.  Always remember that the long way is the wrong way!  Set up your proportion showing acres per gallon and use an x to mark your bottom line.   Then attack the problem using cross-multiplication to solve for the bottom line.

Check back to make sure that the x you solved for matches your bottom line, then look down at your answer choices.

(A)

(B)

(C)

(D)

(E)

The correct answer is (A).

On sat.collegeboard.org, 70% of the responses were correct.

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