table.rb

def is_guy(s)
  if s == 'a' or s == 'b' or s == 'c' or s == 'd'
    return true
  end
  return false
end

def two_guys(p)
  9.times do |i|
    if is_guy(p[i]) and is_guy(p[i+1])
      return true
    end
  end
  return false
end

n = 0
ne = 0
n2 = 0

a = [ 1, 2, 3, 4, 5, 6, 'a', 'b', 'c', 'd' ].permutation do |p|
  if is_guy(p.first) or is_guy(p.last)
    ne += 1
    next
  end
  if two_guys(p)
    n2 += 1
    next
  end
  n += 1
end

puts "n = #{n}"
puts "ne = #{ne}"
puts "n2 = #{n2}"

http://math.stackexchange.com/questions/1142212/can-anyone-check-if-this-is-correct