https://projecteuler.net/problem=2

gistfile1.rb

s = 0

# First two Fibbonacci terms
f_last = 1
f = 1

while f < 4 * 10**6
  # Sum evens
  s += f % 2 == 0 ? f : 0

  # Next Fibbonacci number
  f_new = f + f_last
  f_last = f
  f = f_new
end

puts s