Solution: Finding Prime Numbers

array = [] 
(2..20).each do |num|
  # check if num is a prime number or not
  flag = true
  (2..num - 1).each do |x| # check each possible divisor
    if num % x == 0
      flag = false # mark this has divisor
      break # no point to check more - composite, moves on to next
    end 
  end
  if flag == true # the number has no divisors 
    array << num # add to prime number list
  end 
end
puts "Prime numbers (up to 20) are : #{array.join(', ')}"