euler.rb

# 1
(1...1000).select {|i| i % 3 == 0 || i % 5 == 0 }.inject(:+)

# 2
fib1,fib2,sum = 1,1,0

until fib2 > 4000000
  fib1,fib2 = fib2,(fib1 + fib2)
  sum += fib2 if fib2 % 2 == 0
end

# 5
n = 2520; n += 1 until (1..20).select {|i| (n % i == 0)}.size == 20

# 6
((1..100).inject(:+) ** 2) - ((1..100).inject {|sum,n| sum + (n**2)})