2021 AMC 12B Fall Problem 5

Attempt Problem 5 of the 2021 AMC 12B Fall 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 2021 AMC 12B Fall solutions, or check the answer key.

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5.

Call a fraction ab,\dfrac{a}{b}, not necessarily in the simplest form, special if aa and bb are positive integers whose sum is 15.15. How many distinct integers can be written as the sum of two, not necessarily different, special fractions?

99

1010

1111

1212

1313

Answer: C
Concepts:fractioncasework
Difficulty rating: 1400
Solution:

Listing a/(15a)a/(15-a) for 1a14,1\le a\le14, the integers are 2,4,14;2,4,14; the fractions with fractional part 1/21/2 are 12,32,132;\tfrac12,\tfrac32,\tfrac{13}{2}; and 14,114\tfrac14,\tfrac{11}{4} have complementary fractional parts 1/4,3/4.1/4,3/4.

The remaining fractional parts are 1/14,2/13,4/11,2/3,7/8,1/7,1/14,2/13,4/11,2/3,7/8,1/7, and none of their complements occurs. Thus only the integer, half-integer, and quarter cases can give integer sums.

Integer pairs give 4,6,8,16,18,28.4, 6, 8, 16, 18, 28. Half-integer pairs (12,32,132)\left(\tfrac12, \tfrac32, \tfrac{13}{2}\right) give 1,2,3,7,8,13.1, 2, 3, 7, 8, 13. The quarter pair 14+114\tfrac14 + \tfrac{11}{4} gives 3.3.

The distinct integers are 1,2,3,4,6,7,8,13,16,18,28,1, 2, 3, 4, 6, 7, 8, 13, 16, 18, 28, a total of 11.11.

Thus, the correct answer is C.

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