2021 AMC 12B Fall Problem 25
Attempt Problem 25 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.
All problems are used with official legal permission of the Mathematical Association of America (MAA).
25.
For a positive integer, let be the sum of the remainders when is divided by and For example, How many two-digit positive integers satisfy
Answer: C
Small Hint:
When increases by each remainder rises by unless is divisible by where it drops to
Big Hint:
so you need the divisors of among to sum to
Solution:
Going from to each remainder increases by unless is divisible by in which case it drops from to So
Distinct integers from through can sum to only as or Divisibility closure eliminates every case except for example, a multiple of also has divisor and a multiple of also has divisors and Thus must be a multiple of with no other divisor in
Testing the multiples in the required range leaves only and Hence or giving values.
Thus, the correct answer is C.
Problem 25 in Other Years
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