Math ?
posted by Nikki on .
Which of these numbers is prime? Explain why or why not.
407
409
411
413

http://en.wikipedia.org/wiki/Prime_number
So ... which of these can be divided evenly only by itself and 1?
?? 
Which one(s) are evenly divisible by prime numbers less than the sqrt of the number. The square root of the largest is 20.32. Since they are all odd, 2 is not a divisor, Therefore, division by 3, 7, 11, 13, and 19 are the only tests.