http://projecteuler.net/index.php?section=problems&id=2

p2.rb

#!/usr/bin/env ruby
# -*-coding:utf-8-*-
$VERBOSE = true
=begin
Project Euler: 2
By considering the terms in the Fibonacci sequence whose values do not exceed four million,
find the sum of the even-valued terms.
=end

fib = Hash.new{|hsh, n| hsh[n] = hsh[n-2] + hsh[n-1] }
fib[0] = 0
fib[1] = 1
puts (2..4_0000).map{|i| fib[i]}.select{|i| i.even? }.inject(:+)