aptinio

(1..100).to_a.reduce(:+)**2 - (1..100).to_a.reduce {|sum_of_squares, n| sum_of_squares + n**2}

aptinio

(1..20).to_a.reduce :lcm

aptinio

def palindrome?(integer)
  string = integer.to_s
  string == string.reverse
end

products = []

999.step(100, -1).to_a.each do |a|
  999.step(100, -1).to_a.each do |b|
    product = a*b

aptinio

require 'mathn'
600851475143.prime_division.last.first

aptinio

fib = [1, 2]

def fib.next
  self[-1] + self[-2]
end

fib << fib.next while fib.next < 4_000_000

fib.select {|f| f.remainder(2) == 0}.reduce(:+)

aptinio

(1...1000).to_a.select {|n| n.remainder(3) == 0 || n.remainder(5) == 0}.reduce(:+)