2001 AMC 12 Problem 7

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Concepts:Diophantine Equationprime factorizationbounding to limit cases

Difficulty rating: 1370

7.

A charity sells 140140 benefit tickets for a total of $2001.\$2001. Some tickets sell for full price (a whole dollar amount), and the rest sell for half price. How much money is raised by the full-price tickets?

$782\$782

$986\$986

$1158\$1158

$1219\$1219

$1449\$1449

Solution:

Let nn tickets sell at full price pp dollars. Then np+(140n)p2=2001, np + (140 - n)\dfrac{p}{2} = 2001, so p(n+140)=4002=232329.p(n + 140) = 4002 = 2 \cdot 3 \cdot 23 \cdot 29.

Since 0n140,0 \le n \le 140, we need a factor of 40024002 with 140n+140280.140 \le n + 140 \le 280. The only such factor is 174=2329,174 = 2 \cdot 3 \cdot 29, giving n=34n = 34 and p=23.p = 23.

The full-price tickets raise 3423=78234 \cdot 23 = 782 dollars.

Thus, the correct answer is A.

Problem 7 in Other Years