find largest three digit prime palindrome

gistfile1.rb

def prime?(n)
  for i in 2 ... n/2
    return false if n % i == 0
  end
  true
end

def palindrome? n
  s = n.to_s
  s.slice( 0, (s.length+1)/2 ) == s.reverse.slice( 0, (s.length+1)/2 )
end

n = 999
n = n-1 while !prime?(n) or !palindrome?(n)
puts n