2016 AIME II Problem 1

Attempt Problem 1 of the 2016 AIME II 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 2016 AIME II solutions, or check the answer key.

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1.

Initially Alex, Betty, and Charlie had a total of 444444 peanuts. Charlie had the most peanuts, and Alex had the least. The three numbers of peanuts that each person had form a geometric progression. Alex eats 55 of his peanuts, Betty eats 99 of her peanuts, and Charlie eats 2525 of his peanuts. Now the three numbers of peanuts that each person has form an arithmetic progression. Find the number of peanuts Alex had initially.

Answer: 108
Concepts:geometric sequencearithmetic sequencesystem of equations
Difficulty rating: 2050
Solution:

After the eating, 4445925=405444 - 5 - 9 - 25 = 405 peanuts remain, and the three amounts form an arithmetic progression, so the middle amount, Betty's, is 4053=135.\frac{405}{3} = 135. Hence Betty started with 135+9=144135 + 9 = 144 peanuts.

The starting amounts form a geometric progression, so they are 144r,\frac{144}{r}, 144,144, and 144r144r with r>1r \gt 1 (Charlie had the most and Alex the least). Then 144r+144+144r=444,\frac{144}{r} + 144 + 144r = 444, which simplifies to 12r225r+12=0,12r^2 - 25r + 12 = 0, with roots r=43r = \frac{4}{3} and r=34;r = \frac{3}{4}; since r>1,r \gt 1, we take r=43.r = \frac{4}{3}.

So Alex initially had 14434=108144 \cdot \frac{3}{4} = 108 peanuts. (Check: after eating, the amounts 103,103, 135,135, 167167 increase by 3232 each.)

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