Explanation
In this question, the "number of occupied seats at the
beginning of the lecture" was 135, since there was no further time for anyone
else to come in or leave. And that number is 75 percent of the total capacity:
where we have named the capacity C. We know that the 0.75 goes on the right side of the equation
because, in expressing the sentence as algebra, the word "is" in "135 is 75
percent of" will mean an equal sign.
To solve for C, we
must isolate it. One way to do this is to convert 0.75 into a fraction and
multiply both side of the equation by its reciprocal.
We have found the seating capacity: it's 180. The question
asks for how many were empty two minutes prior to the lecture. At that time,
112 seats were filled, so the number empty was 
.The correct answer is (E).
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