def mean(array)
array.inject(0) { |sum, x| sum += x } / array.size.to_f
end
puts mean([1,2,3,4]) # => 2.5
puts mean([100,100,100,100.1]) # => 100.025
puts mean([-100, 100]) # => 0.0
puts mean([3,3,3,3]) # => 3.0