Aaron Feng

start = Time.now
total = 0
(1...1000).each do |n|
  total += n if (n % 3).zero? or (n % 5).zero?
end

puts "Took: #{Time.now - start} seconds"
puts total

start = Time.now
def fib(n1, n2, total)
  return total if n2 > 1000000 
  total += n2 if (n2 % 2).zero? 
  fib(n2, n1 + n2, total)
end

puts "Took: #{Time.now - start} seconds"
puts fib(1, 2, 0)

def next_prime(start_num, max_num)
  is_prime = true
  prime = 0
  (start_num + 1..max_num).each do |n|
    prime = n
    (2..n - 1).each do |nn| 
      if (n % nn).zero? then is_prime = false; break; end
    end
    if is_prime then break; end
    is_prime = true
  end
  prime
end

start = Time.now
n = 1
biggest_prime = 0
num = 317584931803

while(num != 1 or num > n)
  n = next_prime(n,  num)
  if n > 0
    result = num % n 

    if (result).zero?
      biggest_prime = n
      num = num / n 
    end
  end
  n += 1  
end

puts "Took: #{Time.now - start} seconds"
puts biggest_prime

start = Time.now
result = 0
left = 0
right = 0

(100...1000).to_a.reverse.each do |l|
  (100...1000).to_a.reverse.each do |r|
    temp = (l * r).to_s 

    if temp == temp.reverse and temp.to_i > result
      result = temp.to_i
      left = l
      right = r
    end     
  end
end

puts "Took: #{Time.now - start} seconds"
puts "#{left} * #{right} = #{result}"

def is_prime(num)
  if num < 2  then return false; end
  (2..num - 1).each do |n|
    if (num % n).zero?
      return false
    end
  end
  return true
end

def smallest_factor(num)
  (2..num - 1).each do |n|
    if (num % n).zero? then return n; end
  end
  return num
end

start = Time.now
result = 1 
(1..20).each do |n|
  if is_prime(n) 
    result = result * n
  elsif not (result % n).zero? 
    result = result * smallest_factor(n) 
  end
end

puts "Took: #{Time.now - start} seconds"
puts result