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2006 AMC 12A Problems/Problem 10

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The following problem is from both the 2006 AMC 12A #10 and 2006 AMC 10A #10, so both problems redirect to this page.

Problem

For how many real values of x is \sqrt{120-\sqrt{x}} an integer?

\mathrm{(A) \ } 3\qquad \mathrm{(B) \ } 6\qquad \mathrm{(C) \ } 9\qquad \mathrm{(D) \ } 10\qquad \mathrm{(E) \ }  11

Solution

For \sqrt{120-\sqrt{x}} to be an integer, 120-\sqrt{x} must be a perfect square.

Since \sqrt{x} can't be negative, 120-\sqrt{x} \leq 120.

The perfect squares that are less than or equal to 120 are \{0,1,4,9,16,25,36,49,64,81,100\}, so there are 11 values for 120-\sqrt{x}.

Since every value of 120-\sqrt{x} gives one and only one possible value for x, the number of values of x is 11 \Rightarrow E.

See also

2006 AMC 12A (ProblemsResources)
Preceded by
Problem 9
Followed by
Problem 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
2006 AMC 10A (ProblemsResources)
Preceded by
Problem 9
Followed by
Problem 11
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Want to learn how to tackle those tough MATHCOUNTS and AMC counting and probability problems? Check out Art of Problem Solving's Introduction to Counting & Probability by David Patrick.
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