2007 AMC 8 Problem 13

Attempt Problem 13 of the 2007 AMC 8 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2007 AMC 8 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

13.

Sets AA and B,B, shown in the Venn diagram, have the same number of elements. Their union has 20072007 elements and their intersection has 10011001 elements. Find the number of elements in A.A.
503503
10061006
15041504
15071507
15101510
Answer: C
Concepts:Venn Diagraminclusion-exclusion
Small hint:
Let each set have xx elements.
Big hint:
Use ∣A∪B∣=∣A∣+∣B∣−∣A∩B∣|A\cup B|=|A|+|B|-|A\cap B|.
Solution:
Let xx be the number of elements in each AA and B.B. Also let yy be the number of elements in their intersection.
The conditions give us that 2x−y=2007 2x - y = 2007 and y=1001. y = 1001. Plugging yy into the first equation yields 2x−1001=20072x=3008x=1504. \begin{align*} 2x - 1001 &= 2007 \\ 2x &= 3008 \\ x &= 1504. \end{align*}
Thus, C is the correct answer.
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