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2005 AMC 12A Problems/Problem 4

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Problem

A store normally sells windows at $100 each. This week the store is offering one free window for each purchase of four. Dave needs seven windows and Doug needs eight windows. How much will they save if they purchase the windows together rather than separately?

(\mathrm {A}) \ 100 \qquad (\mathrm {B}) \ 200 \qquad (\mathrm {C})\ 300 \qquad (\mathrm {D}) \ 400 \qquad (\mathrm {E})\ 500

Solution

For n windows, the store offers a discount of 100 \cdot \left\lfloor\frac{n}{5}\right\rfloor (floor function). Dave receives a discount of 100 \cdot \left\lfloor \frac{7}{5}\right \rfloor = 100 and Doug receives a discount of 100 \cdot \left\lfloor \frac{8}{5}\right\rfloor = 100. These amount to 200 dollars in discounts. Together, they receive a discount of 100 \cdot \left\lfloor \frac{15}{5} \right\rfloor = 300, so they save 300-200=100\ \mathrm{(A)}.

See also

2005 AMC 12A (ProblemsResources)
Preceded by
Problem 3
Followed by
Problem 5
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Looking for a challenging geometry text? Preparing for MATHCOUNTS or the AMC exams? Check out Art of Problem Solving's Introduction to Geometry by Richard Rusczyk.
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