2013 AMC 10B Problem 24

Attempt Problem 24 of the 2013 AMC 10B 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 2013 AMC 10B solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

24.

A positive integer nn is "nice" if there is a positive integer mm with exactly four positive divisors (including 11 and mm) such that the sum of the four divisors is equal to n.n. How many numbers in the set {2010,2011,2012,,2019}\{ 2010,2011,2012,\dotsc,2019 \} are nice?

1 1

2 2

3 3

4 4

5 5

Answer: A
Concepts:sum of factorsprimecasework
Difficulty rating: 2180
Solution:

An integer with exactly four positive divisors is either p3p^3 or pqpq, where pp and qq are distinct primes.

If m=p3m=p^3, the divisor sum is 1+p+p2+p31+p+p^2+p^3. The values for p=11p=11 and p=13p=13 fall below and above the interval 20102010 to 20192019, so this case gives none.

In the m=pqm=pq case, the divisor sum is 1+p+q+pq=(p+1)(q+1)1+p+q+pq=(p+1)(q+1). If one prime is 22, the sum is divisible by 33; only 20102010 and 20162016 qualify, but 2010/31=6692010/3-1=669 and 2016/31=6712016/3-1=671 are not prime.

If both primes are odd, then the sum is divisible by 44, leaving 20122012 and 20162016. The factorization 2012=45032012=4\cdot503 would give primes 33 and 502502, impossible, while 2016=45042016=4\cdot504 =(3+1)(503+1)=(3+1)(503+1) works.

Thus exactly one number is nice, and the correct answer is A .

← Problem 23#23
Full Exam

Problem 24 in Other Years