The Sieve of Erathosthenes is an ancient method used to find all prime numbers between 2 and a given upper bound maxNum, inclusive. It works like this:
make a list of all numbers between 2 and maxNum, inclusive
for x = 2 to maxNum
if x is not scratched
for y = 2 to maxNum/x
if x*y is not scratched, scratch x*y
In this fashion, every composite number in the range will get scratched while every prime number will remain unscratched. Return the last number which gets scratched when the Sieve of Eratosthenes is run with the given maxNum.