Question 612114
Let P = price of a new book


Since "Mary knows that a used Biology book may costs about half as much as a new one", this means that we can say


Price of Used Book = Half of Price of New Book


But we know that the price of a used book is $90, so we can replace the left side to get


90 = half of price of new book


Now we can replace the right side with P/2 since this represents cutting the price of a new book in half (cutting something in half or dividing by 2 is the same thing)


So we now have


90 = P/2


Finally, we can flip the equation to get



P/2 = 90



So the answer is choice D

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